How often should you wax or service your skis

I took my hired skis back for free sharpening and waxing half way through the week (very strange ski conditions with ice and powder) and the difference was remarkable. I went much faster afterwards (overtook everyone in my group whilst schussing).

I don't know anything about looking after one's own skis, but judging by this experience I'd say waxing and sharpening were both a good idea :D

Ally