The answer hugely depends on how intensely you work. Using hours as a measure of your productivity is a bit pointless I think. It also matters how the work is distributed in time and what kind of work we're talking about.

I can work all day without my productivity suffering if I take it easy enough, but I can exhaust myself in a few hours too if I work super intensely. Increasing work intensity produces diminishing marginal utility for me. Also the fact that I've accomplished much in the few hours isn't much solace if I'm too exhausted to enjoy anything for the rest of the day.