The majority of universities recommend that students only take on a maximum of between 10 and 15 hours employment per week during term. I mean, that will vary. If you take on too many hours, your work may suffer; but then again without a job uni might not be possible, so you need to weigh them both up. All, I can say is my sister is now in her second year and she finds there is a lot of work, even without a job.