Basically, you study on your own. Padi has a refresher course book. I took SSI. They gave me a copy of the OW test, the same one that the OW students take. I took the test (open book at home). Met with the instructor (a divecon - or DM/assistant instructor), and went over the answers. Drove to the pool, geared up, and went throught a limitted check list of skills to be performed. PADI encourages that one administer an exam, but it is not required.
If you took PADI, the skills covered will be part of the PADI's 20 skils check list, it would be the same, minus 6. Actually, it is skills 1 through 14. So it could be done in about 20 minutes if only 1 or 2 students in the class. I imagine it could be stretched longer.... But again, the focus is on the essential skills - mask clearing, reg recovery, reg/snorkel exchange. None of the stuff like removal of gear above/below water, no weight belt removal, no cesa, no fin pivot, no reg free flow, etc... I could look up PADI's minimal skill required if you like.. I have a check list with it from my DM course.
Just enough to get you comfortable back in the water again.