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.
Since Kylo Ren's slaughter of the Jedi was less than 14 years ago and Luke went into exile shortly afterwards, we can conclude that Luke has been in exile for less than 14 years as well.
The book Bloodline is set six years before The Force Awakens. Kylo Ren was still Ben Solo and being trained by Luke at the time of this book. So Luke was began his exile less than six years before the events of The Force Awakens.