How often should you clean the inside of your PC?

If it a large tower like mine that is on the floor at least twice a year. 

Sometimes.       

Not the best answer but whenever it needs it.  Some environments will need more regular cleaning than others. Its worth checking at least once a year for dust and if the PC slows down/overheats sooner.  I'd say check it before the summer to be safe.

At least every few months check external vents and wipe any dust off, if yhat have a lot consider looking inside more often.

It depends on your environment, but you should open up your desktop & do some dusting (with canned air) on your vents & internal components AT LEAST once a year.

Laptops should only be dusted from the outside as it's generally too difficult for most users to properly disassemble, clean & reassemble it...  especially as it requires removing a lot of screws & a guide to do this properly.

If you live with pets, smoke or live in an area that gets very dusty, you'll likely need to clean your PC more frequently in order to maintain your PC's cooling systems (as you'll want to clean & unclog those vents).

If your computer IS smoking...  it's painfully obvious you haven't cleaned your PC frequently enough (or AT ALL) & likely looking at a massive paperweight.

Every ten seconds.