I only work for about 5 or so hours a week during semester as a private tutor (more when you account travel time to students), plus extra occasional work as an adjudicator through DAV. My situation is a bit unusual though because I live wayyy out in the north east but go to Monash, meaning a lot of my day is spent in the car driving, which personally I found draining on my time and my brain.
As an undergrad I spent a lot of time in the car and this year am getting ready to do so again. Having said that, work is definitely useful to have on your resume. I found that I couldn't have much of a 'life' during semester but that was okay with me. The advantage of tutoring is that it's a generally higher pay rate per hour compared to other jobs, meaning I can work less hours for more $$, and if you work for yourself you can arrange your students however you want (within reason). Some of my friends worked part time in more 'real' jobs but at the expense of attending lectures and stuff. It worked for them but personally it wouldn't work for me. It depends on how you learn, how well you study, your personal living situation, etc.
If you need to keep your grades up though, I echo what everyone else has said and keep your hours low if possible.