I would say 10 hours is a good amount of time to work. 20 hours is doable but really pushing it and you should only do it if you need the money. You will average 5 classes that meet 15 hours a week and then you have to factor in homework (usually another 15 hours) and I would really encourage you to join a club and get involved on your college campus. As a freshman, your workload is not as intense as it will be in later years, so you will have time to have a job. Don't overdo it though. If you feel like it is too much, you can scale it back. It does take some time to get used to the college schedule.
That is going to vary depending on your work load from class but for a full time student fifteen hours is generally the maximum you would want to work.