Let's say I have to fill 24 hours worth of classes (6 hrs for 4 days). There are a maximum of 8 sessions available and the possible classes are lengths of 12 hrs, 6 hrs, 3 hrs or 1.5 hrs (but the 1.5 hrs are only available 1 morning and 1 afternoon per day).

How many different combination of classes can I take?

Also, it can be less than 24 hours if necessary.