Normally once a day. It depends on whats going on. If I am waking up from a night of drinking I have to get off 6-7 times because I am so horny. Also I if I havent gotten off in about 5 days I tend to have a wet dream and thats never fun.
I have found that, at least for me, it helps me focus more and be more relaxed. I played High School golf and before big matches I would sometimes sneak away and get a quick release in before going to play. I was more relaxed and played better as a result. If I was ever on the course by myself with no one around, I would hop into the woods and go stroke off. I just felt it released tension and made me play better. Its different for everyone.