Try the SAME interval as you should replace your air filter. It should be every 30,000 miles. Sooner if your drive in dirt, or dusty enviroments.
If you use cheap gas, Regular Unleaded, I replace mine every 20,000 miles. Its a good idea to replace sooner when using Regualr unleaded because more dirt and debris gets into the gas tanks at stations because they are needed to be filled much more frequently than the Supreme.
So Replace it ATLEAST every 30,000 miles, or 3 years, which ever comes first!