Fellas, how many hours of work a day would you consider optimal?
Anything beyond 10 hours and I feel quite exhausted.

id say 10 hours is optimal for you then.

my schedule (when im at 100%) is get up early (6 am), work on stuff thats the hardest, have a 3-4 hr nap, another 6 hours. break. then work on less difficult stuff until im sleepy again (varies. sometimes it midnight, sometimes its a white night and i take my nap earlier.)
this way i can squeeze out ~15-18 hours of constructive work per day.

but i can do so because i dont work for anybody.

When I am in the zone, I easily pull off a productive 12 hours workday. Other days I could go home after 4 because I get nothing done anyway.

I would say anything beyond 5 is pushing it and not sustainable in the long run. I honestly believe I can do more in 5 hours than in 10.

6-8 is the best amount, backed by actual science. Anything beyond 8-10 will make you less productive on the long run.
You can always do a couple of 80 hour weeks from time to time, but you won't last long by trying to do it all the time.

@fjrevoredo and the same studies lean more towards the 6 if I am not mistaken

Depends on how I'm feeling. If I don't hit the ground running (so to speak), nothing productive happens after hour 4. If I do, I can go from 10-16 hours without realizing it