Is a spell forever?

Question

Looking at the questions and answer to these two questions. Why is the Burrow so cramped on the inside? & Why are there "dilapidated" buildings in Harry Potter? and neither mentions the longevity of magic. I recall several instances of magic failing to last; the money that was spread at the big quidditch game and the broken Cabinet that Malfoy worked on. If I recall correctly when Voldemort died, the curses on several people were lifted. I don't recall if any other spells fell apart with his death.

Is there any canon evidence on the the length of time a spell will live?

Answer 1

We know alot of spells have apparently infinite life time. Permanent sticking charms

“My dear old mum, yeah,” said Sirius. “We’ve been trying to get her down for a month but we think she put a Permanent Sticking Charm on the back of the canvas.

and the spells in Egypt that Bill works for Gringotts to break, which are most likely 3+ thousand years old, bordering on 5k years.

Bill, works as a curse breaker for Gringotts Wizarding Bank.’

Bill’s taken us round all the tombs and you wouldn’t believe the curses those old Egyptian wizards put on them. Mum wouldn’t let Ginny come in the last one. There were all these mutant skeletons in there, of Muggles who’d broken in and grown extra heads and stuff.

Answer 2

There seems to be evidence of both outcomes. In HBP, Dumbledore's freezing charm on harry is broken when he dies whereas Mood's protective spells cast on Grimmauld place last after he dies. So I believe the answer depends on the spell.

Answer 3

If you consider potions to be spells, the answer would likely be "generally no"; I can't think of any potion used in the series that didn't have a definite (or at least fuzzy) time limit.

If you expand to other magical effects, "no" seems to be even more prevalent -- see gilliweed.

It's entirely likely that a skilled wizard (or a group of them) that devotes some time and effort to a spell could make it "permanent" (or nearly so). Surely the Floo Network requires more than just the Floo Powder, for example. Horcruxes and Lily's love protection spell are two other examples of (near-)permanence, and the magic used to animate paintings and photographs lasts for an exceedingly long time, as well.

It seems to me that the answer depends on the scope of what you consider "spells," and how long is long enough to be "permanent" or "forever."