The average liberal arts or humanities class (English, sociology, nutrition, art appreciation, etc.) is considered an average 3-unit class. Under most circumstances, a course load of at least 12 units (4 of these classes) is universally considered a full-time class schedule in college.
Science courses with labs (anatomy & physiology, chemistry, microbiology, etc.) are 4-unit classes at some schools, and 5-unit classes at other institutions. This is because you must attend a lecture class in addition to a practical lab class during the same week.
Here's an example of a full-time 14-unit class schedule:
Anatomy & Physiology lecture: Mon & Wed 8am-9:30am (3 units)
Anatomy & Physiology laboratory: Mon & Wed 10am-11am (2 units)
English 101: Tues & Thurs 8am-9:30am (3 units)
Nutrition: Tues & Thurs 10am-11:30am (3 units)
Growth & Development: Tues & Thurs 12pm-1:30pm (3 units) ________________________________________________
GRAND TOTAL: 14 units
You'll notice that the class time per week corresponds to the units. For example, you are spending 3 hours of class time per week in Nutrition, because you will be attending two classroom sessions from 10am to 11:30am weekly. These two sessions are 1.5 hours each day (Tues and Thurs), which adds up to a grand total of 3 credit hours (3 units) per week. That's why it is a 3-unit class. You will typically attend these classes over the course of a 16-week semester, either from January through May, or from August through December. Some people can handle 8-week college courses or even the 2-week minimesters.
By the way, an average bachelor's degree requires 120-130 completed units. The typical associate's degree requires 60-70 completed units.
I hope this helps. I, too, was baffled by the college experience at one time.