Home gaming PC's, never...
The kids Work/UniPC's, working data monthly, archive yearly.
Laptop's - maybe yearly.
Work PC's: I work both at home & the office. Every time I leave one site I synchronise my working data to a USB stick, then resync that to the other site when I get there. At the office, I have daily data synchro to an external USB HDD, which every week backs up to another external USB HDD.
Once a month I back up the latest data to DVD's - 2 sets, one set stays at work, the other lives at my home.
Every 6 months I ghost drive images to another external 1TB HDD which is stored offsite.
Im finding that most PC's when not used for games/video/music archiveing don't require much backup space & a 1TB external USB/ESata HDD is a good cheap solution.