Like the others, it depends.....on how often, how horned up, alone vs not alone, etc. Usually it's a couple of table spoons.
But then there was the time I jacked off with my nephew. He'd been staying with me for about 4 weeks and got to the point of watching some porn together. He was on the couch and I was in a chair sorta looking over his shoulder. He's pretty hot looking. When I came, it was like in buckets all over my neck, chest and pubes. I can't recall ever coming that much or that powerfully. I attribute it to the tension in the air air that had been building up. It was sorta like edging. It was a fun mess!