It won't happen anytime soon... both franchises are cash cows for their respective owners... a lot more money than Disney could afford to offer them. The rights contracts for both Spider-Man with Sony and X-Men/Fantastic Four with Fox both state that at least one film needs to be made every 2 or 3 years or the rights will be forfeited back to the original owners (aka Marvel). That's why you see the Spider-Man franchise and X-Men franchise constantly rebooted and the sequels beings made back to back to back... Daredevil, Ghost Rider, and Punisher rights all reverted back to Marvel last year after no movies had been made for the number of years required. Fantastic Four will be reverted back if they don't produce the reboot that's in the works by sometime next year.
Now, what could happen is that Sony and Fox could lease the character rights to Marvel/Disney for appearances in their films... it's already almost happened with Spider-Man as they showed the Oscorp building in the Avengers... they had to pay a pretty penny just to show that... and talks are taking place with Sony to feature Spider-Man in Avengers 3 most likely... but Fox has completely refused any talks to do with the X-Men... and as long as they continue producing the movies then Marvel will not get the rights back.
I would much rather see Marvel have all the rights as they produce much better movies... but I can't see it happening anytime soon.
My 2 cents.