Clean them:
1) When first acquired
2) when they get noticeably dirty or inaccurate or start to jam or suffer various 'failure to....' conditions
3) when they're getting put away for the season, with some extra rust preventative
4) when they're coming back out for the season, to get off the extra rust preventative
5) every once in a while, just because

Note that you should maintain the outside more often, unless you like rust.

Also, note that 'field strip' basically means 'get it disassembled to the point of being able to run a rod through the barrel.' This can range from 'remove the bolt' in a bolt action to 'remove the recoil spring, barrel bushing, barrel pin, slide, then finally the barrel' from a 1911.

The next 'holy war' to incite: How much oil on frication points? I've heard opinions from 'barely any' to 'dunk that sumbitch in a barrel, shake it out.'