Like Pelafina says, time taken is different for everyone. But there are things you can do to make sure you get the most out of whatever time you
do spend studying.
One thing you really want to do is figure out how you learn best. Some people learn by reading, others by actively solving problems, etc. I'm a very visual learner, so I've always focussed my studying and exam revision around things like colour-coded notes, and making summary posters of different subjects. There are simple online tests that can help you assess how you work best:
LEARNING STYLES:Find out in 2 minutes
And yeah, explaining a subject to someone else is a fantastic way to make sure it's clear in your mind. Find a friend doing the subject you're studying, and take it in turns to teach each other sections of the course. Or try teaching something to your mum or dad, if they have time to listen and help you study.
Also, studying for long, uninterrupted periods isn't usually very productive. Generally people do far better studying for periods of up to an hour and separating them with 10-15 minute breaks.