I took 6 months off after a bad case of burnout and I think you are spot on at 1 year. About 6 months after going back to work (and clearing my plate of a lot of superfluous BS) I finally felt like I was beginning to hit my stride again. It was a big financial hit, but in the grand scheme of things it would have been far worse on my long term career prospects had I not taken that time away. Six months was, in my opinion, the bare minimum for the kind of burnout I was experiencing.