Is there any evidence of Fox and Marvel coming to an agreement for characters from both the Marvel Cinematic Universe and the X-Men franchise to appear in each other's films? If not, when does Fox's license on the X-Men run out?
Here is some info, although I don't know how reliable it is:
"Alright, Gambit1024! I understand why Spider-Man and Wolverine can't be in the Avengers now, after all! But there's still one thing I don't get: Can Marvel/Disney get the rights to those characters back?" They sure can, little voice!
See, the contracts for say, Daredevil and the FF, say that "If Fox doesn't make a movie without these guys in it after this many years, the rights go back to Marvel." Easy squeezie, right? Wrong. Since Fox cares oh so much about the money these potential cash-cows make, (and who can blame them?) they will do everything in their power to keep these names until they realize that people will not pay any more money to see them. That's exactly what happened with the Punisher, and now he's under the Marvel/Disney roof with all of his old friends.
While the above forum post may not be what is usually considered a "reliable source", about a year and a half ago it was announced that Daredevil might go back to Marvel:
Fox is looking for a director on its other major Marvel reboot, after David Slade just dropped out of Daredevil. The studio has a script it likes, but has to get the film into production by fall or contractually risk having the the property revert to Marvel Studios and Disney. Slade could not make the timing work as he is directing the pilot of the series Hannibal.
Director Joe Carnahan seems to be ready to toss in the towel on the last ditch attempt to reboot the Marvel superhero Daredevil before Fox’s rights to the franchise expire this fall.
Last summer, Marvel got the rights on Daredevil back (source).
To me, this confirms the above "they have rights as long as they make the movies (which, in practice, means as long as it's profitable to make them)".
As @VedranŠego's answer says, Fox owns X-men rights till they make movie on them. Fox already have lost rights of Daredevil and related property, so they are not seen interested in losing anymore rights of Marvel character.
Some rumors suggest Fox has to use this property and make film in every 3-4 year too sustain rights. And currently, Fox didn't stop making film on X-Men character because they have a milking cow and didn't want to loose it.
X-Men: Days of Future Past is coming this year and X-Men: Apocalypse, X-Force and The Wolverine sequel are in development. There might be a plan for Deadpool Film but no updates have been given since 2011.
There is also a plan of reboot for Fantastic Four film, which will share continuity with X-men to make a Shared universe like Marvel's Avenger. (source)
So it looks like Fox is not going to loose rights soon and after the The Quicksilver Controversy, Fox and Marvel is not showing any intention for friendly relation and having different version of same character will going to make thing more complex for crossover.