At least 2 metres and if near end of bolt all of it. The fabric may not be available again and you don't want to be short. Some tomes I have purchased 8 metres for a backing for a quilt.
Look at the amount required for a backing the size you use most and keep either in your head or in your purse. If coming to UK we measure all in metres so ensure you have the knowledge . 40 inches equals 1 metre.