Students can go at their own pace, but most courses are supposed to last a few weeks, and require 3-6 hours a week.
EdX is a great resource for students with specific interests looking for instruction beyond the High School level. They provide access to numerous college level courses (from real universities) across a variety of different topics. Courses range from introductory level to advanced. Students can select individual courses, or programs which group related courses together. Students can go at their own pace, but most courses are supposed to last a few weeks, and require 3-6 hours a week. These are often offered for free, but with the option to gain paid access to graded assignments and extra resources.