It does depend to a certain degree on the subject of your PhD. I'm doing a Chemistry PhD which works out at 18-20 hours a week on a spectrometer, 3-5 hours a week in the lab, 8-10 hours processing data, 10+ hours a week reading/report writing/presentations, 3-5 hours a week in meetings/seminars. If you don't have lab/practical requirements I could see 35 hours being approximately enough maybe.