i was doing 30 hours of work as well as 4 (short) days at uni for a bit last year, and while i managed to pick up a few Ds I would NOT reccomend it. I was tired, irritable and stressed, even when there was nothing to be stressed about (although there usually was). I'm aiming for 15ish hours next year, and def wouldnt go higher than 20. Also I didnt even really use the extra money, i believe it filtered into my drinking and clothes buying money. And then I earned to m uch to get all of my tax back
edit: a day at uni for me equates to at least 4 hours of travel for me though, so if you dont have to travvel far to uni maybe doing more than 20 hours would be more maneagable