I had to replace a graphic card once. Maybe because it over headed from the dust. It was expansive.

Whenever there's noticeable dust inside.

i clean mines about every 1-1.5 month. the inside of my case is very clean, then i sprayed it to only realize how much dust was actually in it. get compressed air cans, they will be your best friend.

Be carefull when you do clean it, some people ruin fans or even send static charges through there machines when cleaning and then wonder why it doesn't work when they try and boot up.

Air compressor in the shed 4 me

I wait until the temps of things start to noticeably go up, then I clean out my heatsinks and fans.

I have always cleaned my system using a can of air duster. Turn off the system while cleaning and make sure it's switch off and even unplugg. Dust can cause damage to the sensitive components. I have a 250mm side fan on my case so it collects dust faster and I have to cleaned it out every month. No biggie it only take me 3 minutes.

about every 6 months. sometimes longer since I have 2 PCs to clean out.