During Episode VII various important plot points are revealed but never expanded upon. Do we know how long Luke has been in exile?
Less than 14 years
ALBERTO SANZ: Do we know when the Academy attack happened in the timeline? 14-15 before TFA perhaps?
PABLO HIDALGO: Let's not call it a 'Jedi Academy.' But it wasn't that long ago.
AUDREY: wait just to clarify, do you mean that it was more recent than 14/15 years ago?
PABLO HIDALGO: Yeah. It was more recent than that.
We know it was shortly afterwards that Luke went into exile from Han's conversation with Rey in The Force Awakens:
REY: Why did he leave?
HAN: He was training a new generation of Jedi. One boy, an apprentice, turned against him, destroyed it all. Luke felt responsible. He just walked away from everything.
REY: Do you know what happened to him?
HAN: A lot of rumors, stories. People who'd knew him best, think he went looking for the first Jedi Temple.