Take a look at the App that Superior Threads has for the iPhone.
"How much thread does it take?"

K

I only hand quilt and using Guterman Handquilting thread I generally go through just under 2 spools. My quilting is about every 2-3 inches. So if you quilt more closely you'll want more thread. I always buy two spools and have never finished the second on Queen Size quilts. I guess if I were concerned I might buy three to be safe, but shouldn't need any more than that.