I find this depends entirely on what I'm doing. When I was working on a game full time on my own time, 12 hours a day consistently was easy, and things like eat

I find this depends entirely on what I'm doing. When I was working on a game full time on my own time, 12 hours a day consistently was easy, and things like eating, and my girlfriend at the time, were a nuisance. At most of my day jobs, most of the time, I probably averaged less than an hour a day of productive time, with most days being zero.