Off and on sat evening/sunday afternoonish. But I did get extremely bored, very often. So I took way more breaks than I should have.

If you know the material, you could just go to the page, watch a movie/do whatever, come back and skip to the next lesson once the timer is down. That's what I did. But they do throw you a few trick questions here and there. If I remember correctly, the timer does go away once you "read" through the lesson atleast once.