how often do you clean your airguns?

In my opinion, it depends on what airgun / pellet you use. 

Always give a new airgun a good cleaning. They are often shipped with something akin to a cosmoline type of grease in the barrels. This is a good thing - it may have been waiting on a shelf for months and you want the barrels protected from rust. (I usually use Balisol and a Patchworm kit.) 

After that, I clean mine any time the groups start getting bad. On my TX200 that seems to be roughly after a tin of pellets or so. On my Compatto that seems to be at about 800 pellets or so.

In short, your rifle will tell you when to clean it. 

Happy shooting!