Bitewing xrays, which typically are only a set of 4 xrays to look at the back teeth, should be taken at least on a yearly basis. They are taken to view cavities between teeth that will not be visible by simply examining the mouth. These cavities can advance quickly if undiagnosed, so taking xrays regularly can minimize these growing in size.
A full set of xrays should be taken every 3-5 years according to the ADA. People with a history of cavities, gum disease, trauma to the teeth, etc. should have them taken no less than every 3 years. These xrays take more comprehensive views of the teeth and bone and can detect changes in these.