At which temperature and how long should I cook tilapia on the grill?
Very hot, and until it's done.

I'm not being a smartass here, I'm being serious. When grilling (especially fish), you generally want your grill as hot as possible; you want to promote the Maillard reaction quickly, in order to get nice grill marks.

Tilapia fillets are generally extremely thin. A minute or two per side should do it. The secret with grilling fish is to put it on and then don't touch it. If your grill is properly seasoned and your fish is dry (important! you don't want water here, it will make sticking worse) the fish will pull away from the grill cleanly when it is done on each side.