How much roving should you buy?

How do you decide how much roving to by ? Does anyone know of a “yardage per pound/ounce” table ? Or …does one spin the yarn THEN decide what to knit ???

I all depends on how you spin your yarn! A yarn spun lace weight will have a lot more yardage per pound/ounce than a yarn spun bulky weight.

Sometimes I spin deliberty for a project and create a specific sized yarn, other times I spin without a project in mind.