Here are the details for one of my schools so far:
At UChicago, my prospective adviser said that I should expect to have around a month off per year (probably 2 weeks in winter and 2 weeks in summer).
It's probably fairly reasonabl, though it came as a bit of a shock at first but that was because I was used to being an undergrad where I had at least 2 months off EVEN if I included courses during summer quarter.
I'm on a fellowship for my first two years, but I'll still be pushed to produce results (I'm basically being trusted to do a highly ambitious project).