In a recent interview with Empire Magazine, Bryan Singer revealed the time frame that each time line takes place in relative to the previous films in the series:
“It takes place about ten years, give or take, after X-Men 3; and in the past it takes place about ten years after First Class,”
“It’s its own story, inspired by Days of Future Past. She’s the prime facilitator – it’s her power that enables that to happen. But it’s Wolverine who has the strength and the stamina to be able to go back on this journey. Charles is in a very dark hole when we meet him, and Erik has been gathering his power. They’re in a radically different place.”
So there you have it, both time lines take place roughly “10 years later” from the films that proceeded them, or if you’d like to look at it in Roman Numeral terms, a good X years later (see what I did there?).