It depends on how good you are at budgeting your time, and how well-prepared you are from your pre-pharm course work. I am a 2nd semester P1, and I have been working an average of 25 hrs/week while carrying 17 credits (which is a full course load at my school). I haven't had trouble with my grades, though I don't have a whole lot of free time during the semester!
I fit the majority of my work hours in during the week. Unfortunately I can't do homework at my job. I catch up on studying over the weekends. Luckily my job is flexible so if I have a super-busy week school-wise I can cut down on my hours, but I think my lightest work-week last semester was still about 16 hours.
As far as other people go - I have talked to P3's who have managed to work 20-25 hrs/week all 3 years, and I hope to be able to do that as well. I would guess the majority work around 8 hrs/week, but I know people who do not work at all and still struggle...so unfortunately, it is very individualized.
Best of luck!!