In the fall, I always worked about 30 hours/week with 15-17 credits. I played softball, so my spring was a little different; same credit load, but in Jan and Feb we had practice so I worked 20 hours, then in March and April (when we had games) I could only work 10-15 hours.
Now, I have gone back to complete pre-reqs while working full-time. I use my vacation time for classes. Not fun, but necessary.
I definitely think that schools will take this into consideration. If you can do well in school while working, it shows good time management. Also, you gain a lot of beneficial skills in the workforce.