Basically, pretty much all deals for the rights to use an IP for a specific purpose are written that way - I'm not sure if there's a specific universal time limit or anything, but when you license the rights to use a property (rather than outright buying the IP permanently) most of the time the deal specifies that you need to do something and/or keep doing something with that IP within a specific amount of time in order to continue to hold those rights.
For films, when you want to use an IP, you "option" the rights to the IP for a percentage of what the actual cost of the rights (established by the actual IP holder) would be, and then you have a certain amount of time to get your project financed and running (at which point you actually pay for the full rights) before that option expires and the rights to the IP become generally available again.
Because so many things get optioned and then end up in development hell for years or even decades, a lot of times folks end up having to just throw something out there in order to keep somebody else from using the same IP.
I'm sure Snarf can probably write a book on the subject (or possibly has, lol).