Tom,
I just played about 15 minutes on my nylon. Doesn't seem to be a problem, except for sitting in that position for a while might make the collarbone a little more sore.
What styes do you play and what type(s) of guitar(s). I personally have 3 nylon classical, 1 Taylor 810 acoustic, an American Strat and a Carvin 5-string bass.
I played the bass for church regularly before OHS, and I can imagine it will be a while before I can handle several hours with it strapped on me.