This is so, so, way not a new topic. I only say that because I can remember my father brushing the dust and dirt out of enormous console vacuum tube TV sets. "They work better when they're clean", said he! (Well of course I asked him why he was doing that).
Fast forward to today. I have 2 cats, who have 2 litter boxes, with well over 2 kilos of clay clumping litter in them. Plus, it's a small house, and they're litter scratchers. It never seems to be arranged quite right for either one of them.
Anyway, it still depends on the machine that you're asking the question about. My eMachines with a Prescott P-4, probably needs the most attention. It idles close to 50c even clean. Stock pre-built computers are mostly in Matx cases, with no front fans. Not good. I've been considering stuffing the eMachine guts into a low end CM "Elite 340" case. (ATX chassis but fairly small mid tower). It has a front fan mount for a 120. That'd probably help more than blowing the stupid thing out every day and a half. As dumb as it sounds, I'd actually prefer it live its life out in the OEM case, so I can rub raybay's nose in it. We've been together going on 8 years now, "E" and I.
If you can believe the sensors, the machine I'm on right now is under 100 F(!) at all points, even the CPU. It's a G-41 / Pent dual Core E6300 combo. Hasn't been cleaned in months. It's mATX, but has a 120mm front fan, dialed down a bit. I'll blow it out when I can get the Windows open this spring. No sense doing it now. It's next to the cat litter, and it would just suck the dirt back in when I fired it back up anyway.
So, the more crap you have in the case, and the hotter the machine runs to begin with, is a large factor in deciding how often you should clean it.
I'd say good cable management and good fans, in a good case are at least as important as cleaning intervals.
I tend to put mainstream builds, (even IGP) into good gaming cases anyway. My latest effort is a Core i530 (stock clock), H55 affair using the IGP. It's in a CM Storm "Scout". It doesn't get too much above room temp. Again, that approach manages temperatures much better than cleaning out too much computer, stuffed into too little case, every week or so.