I cast on the mitered square with 30 stitches for each side. Sixty stitches total for the entire square. I am using up about 2 skeins for every 3 squares.
July1st, 2013: I had estimated that I would need 15 but actually ended up needing 13. The estimate of 15 included the i-cord on the edges but I did not use the main color for the edging.

I had four inches of yarn left for the edging. So someone with a looser gauge than me might need two skeins.