I'd say most people take less than a week to prepare. But it depends on how well your school prepared you for these kinds of exams. We had OSCEs for pretty much every rotation and then at the end of 3rd year we had a practice CS complete with note-writing stations between each SP station. I felt very prepared going into CS and I just took 3-4 days before my test to read through FA for CS and practice a couple scenarios with the timed note writing. Some people may need more time and more extensive practice, it's really dependent on the individual.