Also the biggest thing we have to keep in consideration here...is that we only know they're adapting "Days of Future Past", we don't now the how
Obviously they're most likely not going to do a straight up adaptation considering the state of the timeline and universe right now. I suspect the first movie will jump start or "seed" events that lead into the conditions that create the "Days of Future Past" future at some point in later films.
I could be wrong, just speculating after all...but one of the reasons why I didn't really believe the story at first was because of where we are in the timeline. The actual comic story happens when the X-Men are already established. We just started to see the start of that with "First Class". So there are a ton of interesting things that Singer and company are going to have to do. It should be a fun ride to see what they come up with