For your first question, it's really up to you. If you feel like if you've put as much as you can into an exercise then move on to the next, if not then push more. No one can really tell unless they look at your previous work.

For your second question, this is the reason why critiques are important. It can be hard to tell what you're doing right or wrong on your own, especially in drawing when improvement is usually very gradual. The 50% rule is also useful for this because drawing outside of exercises and applying them will show you how much you've changed.