And the answer is about 4 weeks. The internals of your engine, transmission and rear end are bare metal surfaces which are highly susceptible to rust. In fact, they can cease operating effectively if they begin to get even a slight amount of surface rust. They are bathed in oil during normal operation and oil prevents rust. When the car sits, the oil drains off of some of these surfaces.
High humidity accelerates it.
Some so-called 'experts' advise people who don't drive their cars throughout the year to start them up periodically to circulate the fluids and, thus, keep the internal bare metal surfaces coated with oil. While this might seem reasonable, it is the automotive equivalent of the 'old wives tale'. A car in short term storage needs to be driven at least once a month. It requires more than just getting the water temperature gauge off of the peg. You need to take the car out for a drive for about 20 miles or a half an hour.