math is all about repetition. the more you do it and use it, the easier it gets. but burning yourself out by putting in to much time is no good either.
there is not a one sized fits all answer, do what's best for you.
I'd say 20 minutes to 4 hours, but it really depends on what you're learning. For example, if you're having trouble with a math subject, you may want to divide it in half and practice a little each day.
