It really, really depends on your university, the nature of your GRA, and your supervisor.
In my first GRA I was technically an employee of the university, paid a salary, and technically also governed by the university’s part-time officer rules re: vacation time. In practice, none of that mattered; I arranged my days off with my supervisor/PI. My supervisor (and most of the faculty at my department) were thankfully real people who had families and lives outside of work, so he encouraged taking time off. I think I went home twice for about a week in my first two years. I definitely took Christmas and Thanksgiving off; we weren’t expected to work through the entire break (although if you have 4 weeks off of school for Christmas, you might get one week off but work the other 3). I took off EVERY Christmas and Thanksgiving for the 6 years I was in graduate school; I didn’t always go to my parents’ or other relatives, but I wasn’t working. Nobody worked those days! However, like I said, I never took the entire 3 weeks. At most, I got a week, but usually I just took a couple of days. I never took spring break off, though. That was a prime week to get some work done, because I didn’t have to teach or go to class.
So whether or not you are expected to be in the lab 7 days a week, 10 hours a day really depends on your PI. There are some PIs out there like that, who want to see you all the time, and who assume that if they don’t see you you’re not working. Then there are PIs like mine, who wanted to see products - if I turned things in when I was supposed to, he assumed I was working, and I could work from anywhere, including my parents’ home or the beach or whatever.
It also depends in part on the university - some universities do govern GRAs like any other employee, and you accrue vacation, sick time, etc. If you have a grad student union you are more likely to be governed by that. However, a lot of PIs will completely ignore those rules. Or, you might be grudgingly allowed to take the time, but your PI’s estimation of your work ethic and quality as a student will be affected. May or may not be right - but that’s the case.
On that note, I think 2 weeks are normal (spread out), but 3 weeks might be pushing it a little. But it’s only 2 years. You have to put your nose to the grindstone for two years, but after that you can do what you want.