After each shooting session. You should also clean it after every 500-1,000 rounds if you are on an extended shooting session. My answer to every 'how often' question is always the same. By completely, I mean that you should remove the gas tube and piston, clean those, the bore, the bolt, carrier, and related assemblies. You don't necessarilly have to take the action out of the stock unless you get it wet or you're 'meticulous' about cleaning the gun. To review, if you shoot more than 500 rounds in a session, you might want to do a 'quick clean' of the bolt and bore before continuing to shoot. When you get it back, take off the receiver cover and gas tube and thoroughly clean what you can get at. A good after-session cleaning job takes me about 20 minutes not including the initial soak I give the barrel by running a saturated patch through and waiting about an hour.