For me it’s around 8 real working hours on average. Some days more, some less.
I run a few different things, so it really depends on what’s going on that week. There are days I’ll do 10–12 hours, but because I actually enjoy what I’m building, it doesn’t feel like a typical “clock in, clock out” job.
Of course I’m tired at the end of long days, but it’s more a good kind of tired. When you care about what you’re doing, the hours don’t hit the same way.