Depends. It depends on the type of steroid (potency of the steroid), the location on the body being treated (face vs trunk), and the condition being treated. Most of the time it's important to use the medication as perscribed by your dermatologist/doctor until the condition is under control and then to slowly reduce the frequency of use or stop completely. Important to ask your doctor to avoid problems.
Varies. As long as necessary. For a temporary condition less than a week. For chronic conditions several weeks if necessary.