The length of time to finish a course and get a certificate depends on the course. Each course has a different length. The credit hours each course is worth correlates to the runtime. A 1.00 credit hour course is generally just over 60 minutes (most are 60-61 minutes, and a 1.50 credit hour course is generally within a few minutes of 90 minutes.
Most states use a 60 minute credit hour, however, a few use a 50 minute credit hour so there are some variations by state.
A good rule of thumb: A 1.00 credit hour course is about 60 minutes. So if you need 12 credit hours it is going to take around 12 hours to finish your courses.