If it's like most other film licensing contracts, the rights wouldn't revert back to Marvel for seven years after filming wrapped. So assuming the movie bombs and doesn't get a sequel, it would be probably around mid to late 2021 when the rights would revert back to Marvel, assuming Fox doesn't try to make a sequel or reboot it yet again before late 2021. If they do that gives them another 7 year window as long as production was completed on anything filmed. Hence this reboot. Fantastic Four 2 came out in 2007. To retain the FF rights, Fox had to get a new FF film into production fast, which they did with pre-production in late 2013 and early 2014, and filming that wrapped a couple months ago.
So yeah, late 2021 at the earliest, more likely 2023 or 2024 if Fox does a sequel (which depends entirely on how this movie fares), but recent indicators seem to be painting it as a stinker.
As long as production/plans for a new FF movie haven't started by that time, and filming started by a specific deadline established by the time lengths in the contract, Marvel will get the FF rights back sometime in the next 10 years.
Even if the movie doesn't get released? They keep the rights just by virtue of having worked on the film?
That seems like a weird rule. So can Fox hire a couple of film students to make a movie for 100$ every seven years just to keep the rights? Or is it stipulated in the contracts what "producing a movie based on the franchise" has to look like?