I received my MS in CS a year ago this month and it took me 5 years to do it. I had to take pre-reqs and was taking one class a quarter as I was working full time. That being said, something that is very important is SO buy in. There were times I was spending 40+ hours a week on homework/lecture while working 40 hours a week. My wife was supporting and understood that I had to not spend as much time with her as she would have liked because I just didn't have time.