Depends on how much use they are getting
Normally we will clean once upon purchasing and not clean them again until they begin to show a build up of a dirt. May be once a year may be over 3 years. Cleaning them too often could possibly fade the colors, depending on the type of chips you have and what you clean them with
Even with heavy play, I doubt you'd need to clean poker chips more often than once every few years. With light play, you might only have to clean them once, especially if you've got a few sets in rotation.
I've been talking dirty to mine for months and they're still pretty clean. I must be doing something wrong.
But I wouldn't expect you to have to clean them more than once like @allforcharity said. Especially if you're playing monthly and/or have a couple of sets to rotate in. And if you don't have greasy slices of pizza in play, they really shouldn't get too gunked up at all.