many universities offer classes between 7am and 10pm. You register or enroll in the classes you want, on the days you want. the number of units/credits often reflect how long you'll be in class a week. At my schools, a 3 unit course would meet twice a week for 1.5 hours each or once a week for 3 hours. many schools also offer online classes that really only have deadlines for assignments but no "hours."
I used to take classes around my work schedule. I'd have morning classes before work and evening classes after while I was in college. Now that I'm going back I am taking my classes online so I can have more flexibility and not have it interfere with my current job.