I think it depends entirely on your field of work. My work is laboratory-based, and so the hours are less flexible. I probably spend 9.30-6.30/7 in the office/lab on weekdays, and work most of Sundays at home too, so about 50ish hours per week, but this can rise to 60-65 hours per week (with evening work too) if I have deadlines to meet.
The best thing to do would be to speak to colleagues who might have done a PhD, especially if in a similar field. For example, if you're doing something where you have to take readings of something at specific time points, you might lose flexibility. However, I think that even the most rigid of PhDs would offer at least the same flexibility as, if not more than, a job.