At least in our university there is not a rule like there is for employees like post-doc, administration staff etc. It depends on the supervisor, some students work from home, while some others are under a control-freak supervisors that expect them to be in the lab everyday including weekends. I very often feel like a low-paid labour force, with no rights on maternity leave or sick leave or anything.
The first year I didn't get any time off, during the second year I probably got 6 or 7 weeks, and now as I am supposed to complete in a few months I think I will get some time off after a submit a first draft. I am quite worried about reminding myself that life is good outside of the office :)