Don't worry about how long you spend on a lesson. Just do each exercise the best you can and fill AT LEAST the required number of pages. Feel free to do more. After you've done that, go onto the next lesson, but don't stop practicing! Keep practicing these skills over and over,
REPITION IS KEY. Art is just like weightlifting. You won't see amazing results at first, but over time you will build up your skill to a high level. One word of warning- if you are like me, you might get down over perceived lack of progress. Realize that everyone starts out shitty, and you just need to keep practicing to see progress. Sycra has a good
discussion of this.
A lot of people talk about how long professional artists practice for, or ask how many hours you should be practicing a day. I think it is best to not worry about that. Just start doing it, try to do a bit more every day, and build up the time you spend drawing. Pro artists practice crazy numbers of hours, but they built up to that, and that is how you'll do it too (if that is what you want). And a smaller amount of FOCUSED practice will push you further than a longer period of unfocused doodling. That said, you do have to practice a lot to become an art ninja. Follow these
5 steps, too, they are good advice (if you can't find life drawing in your area, don't despair, you can use photos and books and maybe a friend will pose for you).
There are lots of online resources like
this/. If you get a chance to draw real people, though, don't miss it!