How much fuel will a 125cc motorbike use?

Depends if the bike is a 2 stroke or 4 stoke and how hard it's thrashed. A 2 stroke racing bike will probably only get 30-50 (max) miles to the gallon (UK). A 4 stroke will get over double that.

Golden rule, if you thrash the f*** outta it, you'll get less mileage.