It depends on the classes/career your persuing, some of them are useless and you may as well just figure out how to cheat because your wasting your life/time on dumb shit you'll never use. Focus on studying the stuff you'll actually need.
For numerical subjects like Mathematics, practice as much as you can.
If syllabus is vast and you know you can't cover it all up, prioritize doing only those which you are comfortable with and work on them really hard...like make it your strong suit! Then give time to the weak links.