how often should i check for dust build up?

[Jupiter]

I've got a coolermaster 430 and I really don't want dust to build up on the gpu fan , cpu fan ect. How often should I clean it? And with what? Thanks

 

[Jupiter]

Thanks guys, and yeah I've got a cat and also my case is on the floor, unfortunately near a keyboard which seems to get dust on it a lot :/ I think my computer is going to be relocated to the top of my desk soon.