It will probably be set in the 80s or 90s, I imagine they already decided on this a while ago especially if mcavoy is in the movie, kinberg has pretty much said xavier will be in it so it's either mcavoy or Stewart, rumour suggests it's mcavoy and he was told he is in it by an actress involved with the project. Stewart apparently believes Wolverine 3 could be his last so he hasn't heard anything unlike with DOFP where he was told he would be in wolverine 3.
One possible reason it will be set in the past is probably so they can use a young sunspot rather then retcon him younger after DOFP especially when it's meant to be a film about young mutants so it just adds up to put it in the past and also I doubt they care or think it matters all that much if it's in the past, it doesn't stop them making movies about these characters which is what this is all about, not about parent day stuff.
It's like wolverine 3, it's very likely to be set after the future ending of DOFP and then another few years after that also, they ain't worried that it should be kept in the present, no they ahead to what kinberg has called a futuristic Western feel.