I think of it as a job and not school. Week for Christmas, likely a week or so in the summer. I left Wednesday evening for Thanksgiving and was back for class Monday.
Generally speaking, if faculty are expected to be working, I expect to be working. Depression/eating disorders/substance use don't disappear for the entire month of December, so I'd feel bad cancelling clients, stopping research etc. at that time. Then again that's my own rule, not really a school policy. Though I think if say, you wanted to leave for a month at a time, it better be due to some crisis somewhere. It also may depend on whether you are funded or not. I think a school you pay to attend probably has less say in how you spend your breaks than one that is continuing to send you paychecks over winter break. Of course, even at those schools if you're slow getting your thesis/dissertation proposal written because you dont work on breaks, they won't be happy either😉
I know even at my school, different people have different ideas of what breaks mean. Some people abuse it, but those people are usually not terribly successful students anyways. I'm probably on the conservative end of things, but I figure its better to plan on this and if I can take an extra week at some point, so much the better.