I work 28 hours while in a full time program with no problem. I will say that I do not have any children, just a husband. I am in my third semester, I did 36 the first semester and then went down to 28 in the second semester when pharmacology came along with nursing classes as well. (still 75 hrs out of the house a week w/ work and school not counting studying or travel!) The 28 seems perfect for me, I can't really do less than that, d/t mortgage payments.
It's all about time management. I plan my study time well and do most of it in the library or away from home and other distractions, because I lose time at home ( laundry, cleaning etc...). I also find the busier I am, the more I actually get accomplished. But that's just me.
Probably no more than 24 hrs a week, 16 would be optimal, unless you really need the money. Best of luck to you!