Fox will release X-Men: Apocalypse on May 27, 2016, the studio (and director Brian Singer) announced today.  That keeps movies from the X-Men franchise hitting around the third week of May, similar to Fox’s release date for X-Men: Days of Future Past, which will premiere May 22, 2014 (see "Reshoots for 'X-Men: Days of Future Past'"). 

Fox must be pretty sure of the results of its next X-Men release, and have deals with at least some of the talent, to be planning a new release well before the current one is in theaters.