A standard session is 16 weeks. In a 16 week session, a four hour course meets four hours per week. Since the recommended time spent outside of class is three to four hours for each hour in class, that means that the overall time commitment for a four hour course in a 16 week session is four hours in class and twelve to sixteen hours outside of class, a total of 16 to 20 hours per week.
A course taken in an 8 week session covers the same material, in the same depth, as the same course taken in a 16 week session. Since it is covered in half the time, expect to put in twice the total time commitment each week. That means eight hours in class and 24 to 32 hours outside of class, a total of 32 to 40 hours per week.
Note that this is like a full time job. Short sessions can be an effective way to speed up your math course sequence, but only if you have a substantial amount of time to devote. Please be realistic about the time you have available before registering for a short session.
Likewise, you can expect other short sessions (5-week, 10-week, 12-week, or 14-week) to require more time than the standard 16-week session.