No worries - just one of those questions that sort of depends on where you want to go with your art. If you're not sure at this point then just touch on all bases and see what you like. I would advise you to have some sort of idea regarding direction and goals. Industrial designers have a very different skill set and awareness compared to landscape painters...who are different from portrait artists...and so on.
As a starting point though you can't go wrong with solid study of the fundmentals, because they apply to all of it. Good luck!
What would Caravaggio do?
I would say as much time as you are able to, in order to see a tangible improvement. But keep in mind that life studies encapsulate far more than just figure drawing. Different shapes, surfaces and areas all serve to train you how to render a 3dimensional form into two dimensions. Alternate your studies with drawing for fun and for the quick results, post your work on the boards for advice and critique.