I think the question you want to ask is slight different from the one you're asking. A lot can depend on the definitions here.
I tend to separate "stipend" from a teach assistanceship (TA), from a research assistanceship (RA), mostly because that's how it was when I went to school. The stipend can be considered money that you get just because you're in the program. It's like an award. For the TA and RA you are required to put in time. The way it worked when I was a student about half of what we made was a stipend and the other half was from a TA/RA, and that was sometimes replaced by additional awards and scholarships. (Note this may vary from place to place since in some schools tuition is waved.)
The TA basically worked out to two 3 hour labs per week + prep and marking time. Officially we were paid for 12 hours. Aside from reinforcing the basics (which is important) that would probably not be considered learning/studying/research time for a graduate student.
It's safe to assume that you'll be working as if at a full time job, and likely doing some studying or marking or reading or other research-related work (including writing papers/thesis, preparing talks, administration) in the evenings as well. Assuming you don't have family obligations, you'll have time for extra-curricular activities. Some people are able to hold down a part-time job or do some volunteer work. You'll get busier as exams, committee meetings, conferences and thesis deadlines approach - some more than others.