AR's (and most gas based weapons I would guess): every 1000-2000 rnds or significant loading/F2F issues
Pistols: 500-5000 .. just depends on the gun
AK's (and most piston type weapons I would guess): When it stops feeding.. eh, call it 5000 rnds
Bolt Guns: About every 500rnds


Most of us who have been in the military have the awful habbit of cleaning them every time we shoot em'. Not needed, not exactly harmful, but it just isn't as important. Now, if you're going to be chasing Hadji in sand, sleet and other inclimate weather, that's a different story.

As always, if you get em' wet, it's wise to wipe the water out..