Wow... I'm a bad person xD
Unless I'm installing upgrades... I Never open them up, and I build my machines so they don't need upgrading for years.
Latest PC has a GTX 780 and i7 4770k so I probably won't be opening it for a good 2-3 years xD.
To be fair, the case is a Corsair 600T Special White Edition so I can see the cool lighting effects and if there is any dust build up without actually having to open it up and the case is fully filtered so it doesn't really get dust build up anywhere.
...
Just checked my old pc which used a Cooler Master Cosmos S case, that hasn't been opened in a good 3 years and there's a light layer of dust on the PSU and the radiator has got a bit on but it's actually not to bad! Considering its been on, a good 12 hours a day, everyday, since it was built.