As per http://copyright.cornell.edu/resources/publicdomain.cfm:
Published Copyright term Ergo, public in Episodes
1964-1977 95 years after pub date 1977 + 95 = 2072 IV
1978-3/1/1989 70 years after author dies ???? + 70 = >2082+ V, VI
1978-3/1/1989 95 years from publication 1980 + 95 = 2075 V, VI
if corporate or 120 years from creation,
authorship whichever expires first
Therefore, Episode IV will be 2072.
I am not sure if Episodes V, VI are "corporate authorship", if so, they would expire into public domain either in 2075 or 2078 (if "authored" by LucasFilm), or 70 years after Lucas dies, which means 100% sure after 2082.
The rest of the years after 1989 fall under the same rules as the corporate authorship for pre-1989, therefore Episodes 1-3 will expire 2094, 2099 and 2102.
Please be aware that I'm using year of release as "publication", therefore that may be off by a year or two if the precise definition of publication for movies for the copyright purposes is different.
As a side note, copyright laws have been changed before, with retroactive effects, specifically with the goal of lengthening the expiration period for works about to expire (I'm looking at you, Disney).
So it's QUITE possible they will be changed again before 2075 to make these dates much further into the future.