Given that this question seems to concern three IPs these days (X-Men, Spider-Man, and Fantastic Four), the question is which of the three would revert first. Of those, X-Men is doing well. Spider-Man had disappointing but still solid returns and the plans to expand the universe appear to be continuing (albeit at a slower pace). Fantastic Four appears to be putting out the minimum number of films necessary to keep the rights. My understanding is they have seven years to start production on a film and a shorter period of extension time to complete it.
The current film is seemingly made relatively cheap compared to other comicbook properties. Therefore, Fox is managing expectations. In order for the rights to revert, it's gotta not only be a disappointment, it's gotta fail to make back its money. Right now, Fox has a sequel planned so they at least have hope of that.
But the question is who will revert first. Fantastic Four is the least successful of the properties and the most likely to revert first. But it'll be at least eight years from now and likely well after that.