I go by the box-full... while I can't really say how much passed through my hands, I can know whether or not everything fits in the existing boxes or if I had to create new ones. No matter what storage size I had, from basically none to a lot, it always has been I've had more fabric than space
So what I've learned is determine the amount you feel is reasonable and keep to it.
Some people use weight. In pounds, it is about maybe 4 yards but how much of that is usable depends on the type of scraps.
I've never really wanted to know my yardage or how much is in one box... but it's a lot! lol I still maintain that there is nothing wrong in even a huge stash if you have both the space and the organization.
For a visual, some 10-15 years ago, I had a "fabric storage room" and since then I have taken about 8-10 full black plastic bags worth of things out of there to turn it back into a sewing room (and it will never make it back to shared guest/sewing room)... I'm still going to have a sizable stash but I think I can be happy with a closet full instead of a room full...