It depends. My general guideline is the efficiency of case extraction:
Marlin 39 TDS: When the ejection becomes sticky.
Ruger Mk I, II, III: About every 300 rounds or the first time it jams, which ever comes first.
Ruger Single 6, SP101, and S&W 617: When the fouling on the front of the cylinder begins to offend my sensibities.
Ruger 10/22's: I don't think I have ever cleaned one of mine.
Depending on how energetic I am (and how much time I have), cleaning will range from a Boresnake and aerosol cleaner to a rod-brush-patch job.
After reading all of the posts so far, it looks like the ejector and chamber need cleaning more than the bore.