A unit represents approximately three hours of work per week. Thus a 3 unit course will probably require 9 hours of work per week, a 5 unit course will require 15 hours per week, and so forth. Of course, the actual hours may vary somewhat from class to class and student to student.
Though a unit is 3 hours of work per week, it is expected that only one of these hours will be classtime. As a general rule of thumb, you should expect to spend two hours studying or working outside of class for every one hour in lecture or discussion. A typical 4-unit course thus will require about 12 hours of work per week: 4 hours of classtime and 8 hours of work outside of class.
If you enroll in 15 units of coursework (3-4 courses), this will will require about 45 hours per week, only 15 hours of which may be class time. For comparison's sake, a full-time job is typically 40 hours per week. To graduate in 12 quarters (4 years), you should average about 15 units per quarter to achieve 180 units, although transfer units count toward completion of these 180 units.
In your first quarter, we advise students to consider the transition to Stanford itself something like a 3-unit course (so nine hours a week). And this is further complicated should you want to pursue research or extracurricular opportunities. We encourage students to think of any activity that they regularly participate in as counting for 1 unit for every three hours. So a 12 unit course load with 6 hours of exercise a week and nine hours of extracurricular organizations is actually the equivalent of 17 units. It all adds up.
You can read about Stanford’s policy regarding units for more information.