Only you know what your learning style and energy level are like. Some people can work a 40 hour job and still take 10-12 credits at a time, Others can barely work 5 hours and still get through their classwork. It varies by individual.
But as you are just taking 2-3 hours less of school, you might be able to add about that same amount to your work schedule. Maybe 15-20 hours a week of work.
Normally there is a set number of credit hours you need to complete your degree. Take that number and divide it by 12. That is how many semester it will take you to complete the schooling. As you did not tell us if this is a two year college, a four college, a trade school, classes for an Associates degree, classes for a Bachelors degree, some kind of a technical certificate or what, we have no idea how many credits you will need.