You might want to reach out to current students in the program and ask them about it because it could vary depending on the school.
I personally would find something that will allow flexible hours/working on the weekend. Or something that will allow you some leeway to study at work. Like I said, it could vary depending on the school - if electives from other departments are offered during the day, you want to do an internship, or there are campus events going on, that may make a difference.
During my senior year of undergrad I took 15 credits, worked 20 hrs/week, and did an internship, but that's undergrad. From that experience I can say it was difficult but doable. You can find out from current students what to expect for how much time you'll need to spend on school work outside of class, which I think would be helpful.