During year 12 I did most of my work in my frees (about 13 hours a week) with a little bit of extra work at home (mostly revision and only a few hours a week). Now in year 13 I finish my homework in my frees, then do about an hour every night of AS revision and going over subject notes. It’s also worth keeping track of how much you’re doing so you don’t too much or too little. As long as you focus and remove distractions while you’re working, you shouldn’t have to spend too long on it.