I know that's a vague question - let's say three-tier cakes. 12", 9", and 6" layers. Or some other way to convey how long that bucket of fondant will last so I can figure out if it's worth the cost versus making my own.
There are sooooooooooo many differences to take into consideration:
How thick/thin do you roll it?
What size(s)/shapes the cakes are?
How many decorations will it be used for?
There are charts around that say how much (in weight) each size cake takes.
I have found that is usually way off. Usually I end up with almost as much leftover as I used when I used the chart.