I just did a 60x72 inch quilt with pretty dense quilting and used most of a 500 yard spool of Tut on just the top -- I used a different color for the bobbins and also for the border. So I would buy at least two for just the top alone.
But I try to buy thread either in very large quantities (colors I might use alot) or at the LQS so if I run out, I can always pick up another spool.