I realise I'm in the minority here, but to be honest no, I don't think it should be too much if you manage your time well. I'm 21 so older and a little more experienced which may help, but I worked somewhere between 16 and 20 hours a week last year, volunteered one evening a week at the local hospital and have a lot of responsibilities at home [looking after 5 year old sister, cooking meals as mum works and dad is disabled and so needs help], and I managed AAAB in my AS levels [maths, human biology, psychology and chemistry respectively]. This year I am working 20 odd hours a week [I have a car and all associated costs to pay, plus other essentials, so not working is not an option] and still helping at home, although I have had to give up the voluntary placement to help my father more, and on course for AAA [I dropped psychology]. I have a reasonable amount of down-time, although granted I don't have much of a social life - my friends moved away recently - so post-shift drinks are my main social thing.
It looks good on a Uni application to show that you can handle work and still get good grades, and I think it sets you up better for life if you can learn to cope. Yes it may mean foregoing the odd party, but time management is such an essential skill to learn.
But, if it is too much for you and you can afford to cut it down, it may well be worth doing so - grades are more important! Personally, not only do I have to earn to pay for the car [which is essential, no public transport at all where I live, so no car, no college, no work, no social life], but I need that extra something to do with my time. I have two jobs, one waitressing [which was my career for 3 years prior to going back to college], and one as a support worker for Mencap, and I love both of them. It gets me out of the bubble of academia, and out there meeting and working with people. Getting a job at Uni will be much easier with my experience, and to be honest, I've worked since I was 16 and would feel at a loose end if I didn't have a couple of shifts lined up a week! If my parents gave me money, and I lived in town so didn't need a car, would I give up work? Tbh I'm not sure I would, the occasional week where there's been no shifts available for me and I've actually felt quite down and at a loose end by the end of it, but that could be just me - I have had the odd issue with mild depression of late, so keeping busy is important to me.
Tl;dr: Ultimately, it's your choice; yes, grades are important, but I personally find that the benefits [not just the money] of working as well make it worthwhile for me. Do what makes you happy, and will best set you up for your future.