Disclaimer: I am not a lawyer. This is not legal advice. Please consult a lawyer in your home country before trying to adapt any of Tolkien's works.
Disclaimer, the Sequel: laws are not written in stone (anymore). I'll do my best to keep this answer up-to-date, but what I've written here reflects my understanding of the law at a particular point in time, and may be outdated by the time you read this.
In the United Kingdom only, with the law as it stands today, individual works will enter the public domain on January 1 of the following years:
- The Silmarillion:
- If it can be considered jointly-authored with both Christopher Tolkien and Guy Gavriel Kay, 70 years after Kay's death; no earlier than 2093, but likely much later than that (Kay is currently in his late 60s)
- If it can be considered jointly-authored with Christopher Tolkien but not Kay, 70 years after Christopher Tolkien's death, in 2091
- Otherwise, 2028
- Unfinished Tales:
- J.R.R. Tolkien's notes will enter the public domain in 2031
- Christopher Tolkien's commentaries will enter the public domain 70 years following his death, in 2091
- History of Middle-Earth
- J.R.R. Tolkien's notes in their individual volumes will enter the public domain in the following years:
- Volume I: The Book of Lost Tales, Part 1: 2034
- Volume II: The Book of Lost Tales, Part 2: 2035
- Volume III: The Lays of Beleriand: 2036
- Volume IV: The Shaping of Middle-Earth: 2037
- Volume V: The Lost Road and Other Writings: 2038
- Volume VI: The Return of the Shadow: 2039
- Volumes VII - XII: 2040
- Christopher Tolkien's commentary, all twelve volumes worth, will enter the public domain 70 years after his death, in 2091
- Any works published prior to Tolkien's death (The Lord of the Rings trilogy, The Hobbit, probably "On Fairy Stories", Leaf By Niggle, "Farmer Giles of Ham", etc.): 2044
- Standalone novels published after Tolkien's death (such as The Children of Húrin or Beren and Lúthien):
- If the law believes that it was jointly-authored: 70 years after Christopher Tolkien's death, in 2091
- If not, 2044
Though bear in mind that these novels are typically very similar to previously-published works (things in History of Middle-earth, for example), so it's not beyond the realm of possibility that they could enter the public domain with those works instead. This would be a excellent opportunity to remind you to talk to an actual lawyer, seriously!
The duration of copyright in the United Kingdom is governed by the Copyright, Designs and Patents Act of 1988. In this case we're interested in the duration of copyright for a literary work; according to Section 12, subsection 2, this is 70 years after the author's death:
(2) Copyright expires at the end of the period of 70 years from the end of the calendar year in which the author dies
Copyright, Designs and Patents Act 1988 s. 12(2)
J.R.R. Tolkien died on September 2, 1973. Unless this law is amended1, all of his published works will enter the public domain in the United Kingdom on January 1, 2044.
This definitely applies to The Lord of the Rings trilogy and The Hobbit, since all four books were written and published during Tolkien's life.
Francis Davey points out a complication in comments: because Tolkien's works were published before the 1988 act came into effect, there is some overlap with the rules of the Copyright Act 1956.
Schedule 1, Paragraph (12)(2)(a) of the 1988 Act lays out when the rules of the 1956 Act apply:
(2) Copyright in the following descriptions of work continues to subsist until the date on which it would have expired under the 1956 Act—
(a) literary, dramatic or musical works in relation to which the period of 50 years mentioned in the proviso to section 2(3) of the 1956 Act (duration of copyright in works made available to the public after the death of the author) has begun to run;
Copyright, Designs and Patents Act 1988: Schedule 1: "Duration of copyrights in existing works" (12)(2)(a)
The proviso to s. 2(3) of the 1956 Act goes as follows:
[I]f before the death of the author none of the following acts had been done, that is to say,—
(a) the publication of the work,
(b) the performance of the work in public,
(c) the offer for sale to the public of records of the work, and
(d) the broadcasting of the work,
the copyright shall continue to subsist until the end of the period of fifty years from the end of the calendar year which includes the earliest occasion on which one of those acts is done.
Copyright Act 1956 s. 2(3)
Translating into English3, we have that works first published (or otherwise made available to the public) after the death of the author are under copyright for 50 years after the date of first publication; this only applies to works published before the 1988 Act came into effect.
In addition, we have Schedule 1 (12)(4)(a) of the 1988 Act to consider:
Copyright in the following descriptions of work continues to subsist until the end of the period of 50 years from the end of the calendar year in which the new copyright provisions come into force—
(a) literary, dramatic and musical works of which the author has died and in relation to which none of the acts mentioned in paragraphs (a) to (e) of the proviso to section 2(3) of the 1956 Act has been done;
Copyright, Designs and Patents Act 1988: Schedule 1: "Duration of copyrights in existing works" (12)(4)(a)
This would suggest to me that works written by an author who died prior to the 1988 Act coming into effect, but which had not been published by the time of the Act coming into effect, are protected by copyright until 50 years after the 1988 Act came into effect. Since, according to 1989 No. 816 (C. 21), the relevant parts of the Act came into affect on 1 August, 1989, I would interpret this to mean that the date of note is 1 January, 2040.
There's a bit of an additional complication with works like Unfinished Tales and The History of Middle-earth, which consist of Tolkien's own notes (lightly edited) and supplemented by Christopher Tolkien's commentary. This is probably sufficient for them to not be considered works of joint authorship, in which case the copyright terms should be based on the lifespan of the person who contributed them.
Assuming that's true, we can apply the rules from the end of s. 2(3) to find that Tolkien's posthumous writings will enter the public domain (according to these rules) on January 1 of the following years:
Christopher Tolkien's commentaries would not fall under this exception to the 1988 Act, and so would pass into the public domain according to the more recent rules.
However, the above does not apply if Christopher Tolkien can be considered an "author" on a particular work. That case is covered in the next section.
There is a slight complication when it comes to some of the works published after Tolkien's death: it's unclear to me to what extent these works can be considered jointly-authored with Christopher Tolkien, who performed some editing on his father's notes before publication.
Section 10 subsection 1 defines what it means for a work to be of joint authorship:
(1) In this Part a “work of joint authorship” means a work produced by the collaboration of two or more authors in which the contribution of each author is not distinct from that of the other author or authors.
Copyright, Designs and Patents Act 1988: Section 10(1)
This definition almost certainly applies to works like The Silmarillion and The Children of Húrin, which were edited into complete narratives from J.R.R. Tolkien's previously-unpublished notes.
If these works are determined to be jointly-authored, then Section 12, subsection 8 reveals that they will enter the public domain 70 years after the death of the last surviving author:
(8)The provisions of this section are adapted as follows in relation to a work of joint authorship—
(a) the reference in subsection (2) to the death of the author shall be construed—
(i) if the identity of all the authors is known, as a reference to the death of the last of them to die
Copyright, Designs and Patents Act 1988: Section 12(8)(a)(i)
Christopher Tolkien died in 2020, so most of these works (assuming they can be considered jointly-authored) will enter the public domain in the UK 70 years after his death, on January 1, 2091.
The Silmarillion was published with the assistance of Guy Gavriel Kay, who is about thirty years younger than Christopher Tolkien, so it's possible that work (or parts of it) could be under copyright for a much longer period of time; it depends on the specifics of their arrangement. It seems likely to me that their contract explicitly denied Kay any copyright over the work, which would make it a moot factor, but those details have not been made public.
MichaelT makes a good point in comments:
Its also possible for the UK to do something like it did with Peter Pan and put it under perpetual copyright or a variant of it.
This is something of an oversimplification of a very special case, which is laid out in Schedule 6 of the 1988 Act:
(1) In this Schedule—
“the Hospital” means The Hospital for Sick Children, Great Ormond Street, London,
“the trustees” means the special trustees appointed for the Hospital under the National Health Service Act 1977 or the National Health Service Act 2006; and
“the work” means the play “Peter Pan” by Sir James Matthew Barrie.
(2) Expressions used in this Schedule which are defined for the purposes of Part I of this Act (copyright) have the same meaning as in that Part.
(1) The trustees are entitled, subject to the following provisions of this Schedule, to a royalty in respect of any public performance, commercial publication or communication to the public of the whole or any substantial part of the work or an adaptation of it.
Copyright, Designs and Patents Act 1988: Schedule 6: "Provisions for the Benefit of the Hospital for Sick Children"
"Peter Pan" is actually in the public domain, and has been since 31 December 2007. Schedule 6 has no provisions for the restriction of adaptation by the rightsholder4; it's just about royalties. It also only applies to performances and adaptations that would be considered "infringing" under the terms of the 1988 Act, or any preceding Act. This is laid out more in Schedule 6 (3), but basically you only need to pay a royalty if your performance would be considered "infringing" if the play were still under copyright.
This has not been done yet for Tolkien's works, and I find it unlikely that it ever will be. As I said, this is a very special case with a complicated history:
- JM Barrie himself transferred Peter Pan's copyright to the Hospital in 1929, eight years before his death
- Schedule 6 was proposed by former Prime Minister Jim Callaghan at the urging of his wife, who served on the Hospital's Board of Governors at the time
- Great Ormond Street Hospital is a registered UK charity, and is the sort of organization pretty much everyone is willing to support
- There are some pretty serious statutory limitations on this right:
(1) The right of GOSH Children's Charity under this Schedule may not be assigned and shall cease if GOSH Children's Charity purports to assign or charge it.
(2) The right shall cease if the Hospital ceases to have a separate identity or ceases to have purposes which include the care of sick children.
Copyright, Designs and Patents Act 1988: Schedule 6: "Provisions for the Benefit of the Hospital for Sick Children"
Which means that the Hospital can't assign these rights to anyone else, and they disappear forever if the Hospital ceases to exist, or stops providing care to sick children
While it's not impossible that something similar could be done for Tolkien's works, it seems unlikely; and even if it were done, those rights would almost certainly be similarly narrowly-defined.
The above only applies in the United Kingdom, and possibly the European Economic Area. All countries have their own copyright legislation, which may or may not make allowances for works published in a different country. If you do not live in the UK, you should look up the copyright laws in your country.
Signatories to the Berne Convention will all have fairly similar terms of copyright (although the Berne treaty only requires copyright to last for 50 years past the author's death), but they will have different nuances (see the tension between the 1988 and 1956 Acts, above). Consult a real lawyer.
Okay, then what?
This isn't the venue to give a detailed discussion of copyright law, but a work entering the public domain means that you are free to do whatever you want with it. Think of everything that's been done with the works of Shakespeare:
And more besides. You will be able to do all of this, for profit, without the approval of the Tolkien Estate. In 2044.
However, you will not be able to adapt any previously-licensed adaptations. Those works are considered separate and distinct, and are (probably; I am not a lawyer) bound by US copyright law. These are complicated issues (moreso since these films are derivative works), and I'm not going to do them justice here. Suffice to say that you're not going to be able to sell your Tauriel fanfiction (legally) anytime soon.
I'm also not sure that you will have the rights to the typesetting of particular published editions; those are covered by different clauses of the 1988 Act, but how and when those rules apply is beyond me at the moment. Consult a lawyer.
1 As Harry Johnston points out in comments, the idea of this term being extended is not unprecedented; the UK has had five copyright laws, each of which extended the term of copyright, and the United States (a very influential voice in international copyright) has a similar storied history of term extensions
2 For reference, s. 2(2) basically just says that the Act applies only to works that were published in the UK or by a "qualified person", defined in s. 1(5)(a); I'll spare you the full definition, but rest assured Tolkien would have been one. Since both provisions apply to Tolkien and his writings, there's no difficulty here.
3 Another obligatory reminder that I am not a lawyer
4 Though, interestingly, the Hospital grants licenses in countries where the work is still under copyright, so this may not be accurate. Once again, I compel you to discuss with an actual lawyer