I have found, depending on what your goals are, an hour or two a day if you have it, with your head actually in a book is about right. Any longer, I tend to go a little crazy. However you dont have to "study" to study Chinese, playing music, films, and Radio etc while you do other things, is all study. There's some good online Chinese radio stations, I just tend to have something like that playing in the background, while I do housework etc.