I did 19 credit hours while working 40+ hours a week, plus commuting 1/2 an hour each way to school. I also coached kids soccer at the same time, plus was married with 2 of my own kids (hence the coaching - they were on the team) I was majoring in mathematics... plenty of homework to do.
I don't recommend it.
But if you do, plan on eating your meals while you're sitting on the toilet... Or, plan on chosing each day whether you'll eat or go to the bathroom.
edit: p.s. I also maintained a 3.85 GPA