Some people say every two years, other go up to five, some don't rotate on a regular schedule at all. When the comb turns asphalt black, its probably time to rotate out that comb. Pesticides (particularly miticides) can build up in the wax over time and may get to a level where it contributes to mite resistance and decreased bee health. Before Varroa mites (and before we started banning pesticides) people would sometimes keep using their comb for literal decades.