In "A Separate Peace" how many years has Gene been away from the school?

"A Separate Peace" by John Knowles, is the first person narrative of Gene Forrester.  Gene has returned to Devon School after a fifteen-year absence.

"I went back to the Devon school not long ago, and found it looking oddly newer than when I was a student there fifteen years before." (pg 1)

Phineas and Gene were best friends.  This novel is a memoir of Gene's years at Devon and coming to grips with the tragedy that occurred there.  He uses this vehicle to do away with the ghosts and guilt that has haunted him throughout his life. 

The story began in 1942. The war was on and young men everywhere wanted to get in the war and go kill Germans.  This was never more true than for the boys at Devon School.  We meet Chet, the school "politician," Leper, the school scientist and quirky kid, and Brinker, Gene's antagonist and judge.  lFinny and Gene were friends and experienced those years at Devon together until the accident, the injury, and the loss of innocence.