Calculations and projections

Careful measurement using a piece of string and a bathroom scale put the GYB (Giantest Yarn Ball) at 63.75" in circumference, and 35 lbs in weight.

The formula circumference = 2πr yields a radius of 10.1451", making the current volume of the GYB 4372.5 cubic inches., with a density of .008 lbs/cu.in.

Assuming the density will remain constant as the GYB grows to it's full 9-foot diameter, it will eventually weight about 5275 lbs, which seems believable.

Yarn comes in standard 4 oz. skeins. The GYB will require 21,100 of these skeins to reach its maximum size.
Wrapping these skeins at 20/day will require about 1000 days, or 3 years.