I'm not in the same situation, but I'm doing my A2's this year, exams are coming up this month. I work in retail in Westfield in London, I started up to 10 hours which like you said is very doable. From January as I got kept on permanently my hours increased significantly, i allowed this to happen as the money was the attraction hear. on £9 an hour working 30-35 hour week I was loving it - at 18 that's a alot more then most will. I was doing 9-3 most days in sixth form then also doing then usually 3 4:30-10:30 shifts weekdays and 2 on sat/sun. Most days I was leaving at 8am and not home till midnight at times. For about a month this was doable as I just managed to stay on top of my work in my free periods however when the coursework started to catch up, then revision for tests, having to write essays I started becoming very sleep deprived and just falling behind on everything, slacking at work too.
March ish I asked them to decrease to about 20 hours a week which is a lot easier to handle, honestly now for me 15-20 hours a week at work is just about right ven now i do get quiet tired, but I've gotten into the rhythm of it and when summer comes it'll all pay off hopefully with getting into uni and being able to pay off all my holdays.
It just depends, do you really need this money/hours? do yu have enough time to produce a high standard of work without killing yourself over sleep and such? Personally think 30+ is too much, but depends how many hours a week you spend doing your uni work