According to IFilm on Dec. 5, director Bryan Singer tweeted about an "X-Men: Apocalypse" sequel for May 27, 2016. This would mean that the release would take place approximately two years after the "X-Men: Days of Future Past" film.
This could be analogous to Thanos of the "X-Men" story line where both similarities come to pass where super hero teams could eventually end up in a mass altercation with a behemoth of corresponding villains of different movie franchises. At this point, considering how the upcoming Bryan Singer sequel premise, this 2016 sequel may follow the comic book story arc of "Age of Apocalypse" wherein there is more time travel involved and an alternative reality the team would have to deal with.
Another situation to consider is that Apocalypse was introduced originally in the "X-Factor" series of comics where Scott Summers tried to reunite their old team. Considering that both Scott Summers and Jean Grey will not be in "X-Men: Days of Future Past", they could be speculated that they could be brought back in "X-Men: Apocalypse".
First of all though, to keep in line with the story, it'd be best to see "X-Men: Days of Future Past" first which being released May 23, 2014.