I ususally scape when the pipe starts to get clogged, or when there's nothing else to smoke
I have a friend who's had his glass pipe "Junior" for like 5 or 6 years and he's never scraped it once. Of course that's because it got cracked from dropping it once and the crack has only gotten worse so that by now the resin is probably all that's holding it together. Stll it's a great pipe.