How often can we race?

From John Stanton's, "Running" (page 379)

For the most part, the longer the race, the greater the stress endured by the runner. For the trained runner, the rule of one day recovery for every kilometre raced is an intelligent guide. A 5K requires one week's recovery; a 10K racers can expect to run one every other weekend; half marathons one every two or three weeks; and full marathons one every month or two.

Your recovery time will be dependent on the intensity at which you race. The faster you race, the more recovery you will require.