That's a good question. I go at least 2 hours out, but I don't expect that to be enough. It is on occasion. I don't like going over 5 hours, because from experience I know guys that book 6 hours or more, away, have a high no-show rate.
Spas know it, so if you book, say, a 4:00 p.m. at 9:00 a.m., and another guy calls at 2:00 for the 4:00 spot, they might give the spot to that caller, thinking he is far more likely to show up.
Moral of the post: if you post, say, 4 or more hours out, call again to confirm, 1-2 hours out. Spas really appreciate when you confirm.