I agree with Sim, although it's best advised to not take advantage of the leniancy in the amount of time you take off, especially if you plan on getting your PhD finished.
I tend to take 2/3 weeks holiday away each year, maybe a week or two off for Christams, and then I have odd days where I either do no work at home, go away, or just have "rest days". I always let my supervisor know if I'm planning a long holiday or planning on taking more than a few days off, but otherwise I'm free to come and go as I please and no-one checks up on me. I work from home quite a bit as well.
As long as you get your work done all is fine, but just make sure your supervisor isn't one who checks up on you if you're planning on taking a lot of time off.