How do you pick out which exercises/etudes to do, and when are you done with them?

There needs to be a good balance between learning new pieces, practicing old/working pieces, and doing exercises. But with the massive amounts of exercises/studies (Czerny comes to mind), each of them taking a lot of time to get it up to a decent level (correct notes, tempo, technique?).

I think some of the problems may be that as a beginner/low intermediate, it's a little hard to see which exercises train what skills, and which exercises are the most important to master. With hundreds of studies in several opus by Czerny/Burgmuller/Heller/Pischna/Brahms/Dohnanyi/Clementi/Schmitt, how do you go about choosing which studies/exercises to do, and when do you decide that you are done with an exercise, and move on to the next one?

Do you ever go back and redo them?