I should actually be working 8h a day, but most of it is spend not working. If I'm honest I'm probably working more like 3h a day even though I enjoy my job.
Back when I did labor work I found I maxed out on efficient work at 6 hours. I couldn't physically do more without increasing my injury risk or doing less quality work.
Then I started doing more office work and desk work. I found I typically would be able to commit 6 hours of honest work before I'd start losing focus and become prone to distractions.