When I was a copier tech we had to go to this gypsum mine at least once a month to clean their copier, it always took half a day to clean out the pounds of dirt. Their computer tech was there one day and I watched him open that thing to about 2.5" of fine dust settled in the bottom of the case, he said that was after a month. He was lucky though, he brought a compressor and just went outside and blew it out, my copier was too big to move.
I've seen lots of home PC's as dirty or dirtier than the ones in those pics (except the one filled with chip bags). Usually when I ask the person how long it's been since their computer was cleaned the response I get is "What, you mean I'm SUPPOSED to clean it out?". I live in the plains where it's always windy and dusty though, so everybody's computers are always dirty.
I'm gonna start taking pics of the bad ones now.