How many time can a person stay in front the computer?
Studying a new language with the computer can be dangerous, because we can stay in it, a lot of hours without realizing the time.
Our eyes put red and we keep reading something new.
I think a person can stay in front of the computer a long time, I spend 60 hours or more in most weeks for various purposes and so far I've been fine :) It can be a bit distracting while studying though, but so are phones...