My PhD is nowhere near as concentrated as my taught Masters, although there are very intense periods to prepare for review points and conferences. I signed what's effectively a contract with the uni which states that I'll work a minimum of 35 hours a week, which is a normal basic working week for most jobs. However I effectively do more hours than that. I'm supposed to take 4-5 weeks of holiday a year - a week at Easter and Christmas, with 2 or 3 weeks (at my supervisors' discretion) over the summer. However in reality I'll be taking less than this. Mainly because I tend to be thinking about my PhD all the time and I don't like having entire days where I do nothing on it. Makes me twitch and I don't find it relaxing.
The way I work is a bit odd. My supervisors have said that they don't care where or when I work, as long as I get the work done to a good standard and meet my deadlines. I'm self-funding and have no input from outside bodies. According to my contract with the uni I'm supposed to work on campus at least three days a week, but there are periods such as writing up review documents or conference material, when I work better from home and don't set foot in the place for a couple of weeks at a time. Some of my colleagues have supervisors who expect them to be at their desks 9-5, Monday to Friday. Those who are doing lab-based research seem to live on campus.
When given a choice, I get up at 7am, work on my sofa (sometimes in front of the telly) until late morning, get into campus at lunchtime, do a few hours there and come home early evening. I'll then probably do a few more hours in the evening. I don't feel like I'm being over-taxed because I'm breaking up my working day.