The Director of Studies for maths told me he expects his students to put in 40 hours a week.....so with 12 hours lectures, 2 or 3 hours of supervisons, that means about 25 hours additional work a week.
So even if you do the full 40 hours, that means about 4 hours a day for 6 days...
or 8 hours a day for 3 days...............
etc.
So not an impossible workload by any standard, and it should leave plenty of time for outside activities.