How long has he been gone?
Well, now that a lot of post-Return of the Jedi material has been released we have a greater sense of the timeline of the fall of the Galactic Empire, what happened to Han and Leia, and how the Resistance was born from the New Republic.
[Updated 11/17/15] Updated with new information from Star Wars: Shattered Empire!
So, what happened to Luke Skywalker after Return of the Jedi?
[Updated 11/17/15] According to Star Wars: Shattered Empire, Luke kept fighting with the New Republic. A few months after the Battle of Endor, he went on a mission with Rebel pilot Shara Bey to liberate a Force sensitive tree from Imperial hands.
It’s a little unclear what happened to Luke immediately after the operation with Bey, but there’s a passage in Aftermath that possible sheds some light on that situation. The passage doesn’t explicitly say Luke Skywalker is in, but gives enough clues that it’s easy to come to that conclusion:
New Republic forces attack and destroy an Imperial academy. A few boys who were intent on becoming stormtroopers approach a man referred to as “The Rebel.” They asked him if he killed “ones who were gonna be baby stormtroopers,” but the Rebel is appalled at the idea. The Rebel tells the boys not to leave home pursuing dreams of fighting for their ideals… but one boy’s parents abandoned them.
The Rebel asks where the boy will go, to which he responds “Nowhere.” The Rebel gives the boy some credits and does his best to help them.
Obviously, the entire exchange is very reminiscent of Luke’s discussion with Owen and Beru about leaving their moisture farm. We can assume that this is not only Luke, but it means that he continued to fight with New Republic forces all the way until the peace treaty signed after the Battle of Jakku.
What happened to Luke after the Battle of Jakku?
It seems everyone was sick of the fighting by that point. Judging from the wreckage at Jakku, the carnage and losses must have been catastrophic. Leia became a general beside Admiral Ackbar, while Han apparently went off with Chewie (though details are still a little fuzzy there).
As for Luke, apparently he went to the planet Devoran, which has a very A New Hope sounding name to it. His goal was to devote himself to studying the ways of the Force and re-establishing a Jedi Temple… though whether this was sanctioned like by the New Republic like the expanded universe’s Jedi Praxeum on Yavin IV we have no idea.
Is that where he is in The Force Awakens?
Possible. We’ve heard rumblings that the location shoot for Skellig Michael featured Mark Hamill, which makes us wonder if that location is Devoran. It certainly screams isolated location for Force training… we don’t be surprised to see it used for such.