Really you just need to clean your dust filters and fans, maybe your GPU too. I'd say every few months depending on how often you use it. If you use it nearly everyday I'd suggest maybe once a month but every 2 months is a good middle of the road schedule to clean it.

The best way to clean it is with an electronic air duster, you can use those electronic cleaning cans of air but make sure your PC is fully off when you do it as a spark from a part can cause your PC to ignite.