It's not known just how long has passed, and based on below, looks like won't be known till more canon is released.
But Pablo Hidalgo in a tweet indicated that it was probably less than 14-15 years, because Academy Attack (which was, presumably, soon after Ren's Dark Side turn) was less than that.
Alberto Sanz ?@Skysawyer 22 Dec 2015
@pablohidalgo Do we know when the Academy attack happened in the timeline? 14-15 before TFA perhaps?
@Skysawyer Let's not call it a 'Jedi Academy.' But it wasn't that long ago. Timeline will be told through stories, not tweets. :)
Audrey ?@natallianovna 22 Dec 2015
@pablohidalgo wait just to clarify, do you mean that it was more recent than 14/15 years ago?
@natallianovna Yeah. It was more recent than that.
Additional probable evidence: Rey is 19 in TFA, and sees herself in flashback as a ~5YO being abandoned. Again, making squishy assumptions, if she was abandoned around the time Ben turned, that looks like ~14-15 years ago. Definitely far from firm or conclusive.
However, take that with a grain of salt - he COULD have turned earlier and hid it. Or turned, left the Academy, spent time training with Snoke, THEN came back and killed everyone. We simply don't know.
Usual disclamer: Hidalgo's twitter account, as usual, has a disclaimer "Don't cite tweets as canon." - and reinforces the reluctance to discuss THIS fact with "Timeline will be told through stories, not tweets". Caveat Emptor.