As the belts ahave dropped in price theres reallyno excuse for doing them, every 2 years for me, which is about 10K miles.
I usually advise about every 10K. you gotta take em off to do the shims anyway (or atleast its a lot easier) so you can give em a good inspection then.
All the modern bikes i think SS onwards use steel pulleys and these are lot more wear ressistant than the alloy ones, so there really little to worry about.
Kev