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."