If working for a company full-time, I'd say 8 hours is too much and is unproductive. You can only focus on a problem for so long before you get distracted and lose motivation.
However, if working on something for yourself you should be pushing 11+ hours a day as a young person. This also means that if you have a fulltime job you should be putting in 4 hours per day after that to develop yourself. That can be working out or learning something new. You cannot depend on your employer forever.