Twice a day at least from when the clocks go forward to when they go back in Autumn. Any more than 3 or 4 a day affects my mood though. In winter though I can go a whole week without needing to cum.
When I was in my teens/ twenties I could do 5 a day easy, and considered that a healthy average. My "Record" was 8, set when I was 18 years old. I tried again in my late 30's to see if I still had it in me. I got my 8 but the last few were no fun, so that was beyond my healthy limit.
What is the limit? I guess if you have a sexual partner and your meat beating is damaging your ability to satisfy them, then it's too much. Likewise if you regularly let your other tasks slip so you can wank. Within that though, there are times when I think it's good to have a "Lost day" and just masturbate and really honour your dick and not answer the door or the phone. Doing this every few months sets me up and refocuses me.