A paper from 2013 published in the Journal of Education and Work (sorry I don't see an ungated version anywhere), by Stephane Moulin (Université de Montréal), Pierre Doray (Université du Québec à Montréal), Benoit Laplante and Maria Constanza Street (both Institut national de la recherche scientifique) provides some answer. The authors use Canadian panel data from 2000 to 2007, and investigated whether the number of hours of paid work affect the tendency for students to drop out of a first university programme. They find:
that there is a critical threshold of number of hours worked, beyond which negative effects in terms of non-completion start to appear... More specifically, working 25 [hours] or more per week increases the hazard of dropping out.This fits well with the prior literature which tends to suggest that there is a U-shaped relationship between work intensity and completing university. That is, the students who work the most, and the students who work the least (or not at all) are the most likely not to complete their degree programme. The explanation for this might be that students who work the most are substituting study time for work time and perform worse academically as a result. Students who work the least might be spending too much time on leisure, or maybe they are just not hard workers (which affects both their ability to gain work as well as their ability to perform academically). Authors of other studies have have also suggested that working might be complementary to study (if work skills or general work ethic are transferable to university), which in combination with substitution would lead to a U-shaped relationship between work and study completion.
I have one gripe with the paper, and that is the causal interpretation that they attach to their findings. They use panel data, and they compare work in prior periods with current study outcomes. However, students that anticipate that they are struggling to pass may increase work intensity, and this effect might be long-lasting. Also, students who are facing financial difficulty may do worse at university (because of stress, etc.) as well as working more. So, I don't think that causality is as straightforward as the authors suggest.
Having said that, 25 hours appears to be the critical value which is reasonably consistent across a number of studies. If you are working more than that, you might want to reconsider your options (I mean your work options, not your study options!).