A lot can really depend on the specific circumstances of the individual, the program, the supervisor, and the type of work you're talking about. Also, remember that a PhD typically takes ~ 4-6 years to complete. So the phase of the PhD will have different demands at different times.
In my experience it was common for students to work a full TA while pursuing the PhD. This involved teaching 3-4 lab sessions (about 12 hours of in-lab time), or marking, or leading tutorial sessions, during the fall and winter semesters. So already most students will have this "extra-curricular" commitment.
My PhD is in medical physics, where many students also took on QA jobs. This could eat up a fair number of hours depending on the details as well, but it was direct, field-specific experience, and it tended to pay well for the time you put in.
Beyond this kind of thing, I'd make sure to have a good conversation with your supervisor or your graduate advisor about a part-time job. You certainly *can* have another job, but remember, that's going to cut into either your study time or other important dimensions of your life (sleep, exercise, social time, etc.). And the effect is that you'll either not perform as well as you might wish, or you'll end up taking even longer to do your PhD. And you may also have specific time commitments to your project. If you have to be in the lab monitoring something at regular time intervals, ducking out of that responsibility to make some cash on the side probably won't go over well.