I shampoo every day, rarely twice a day. And I have long hair, way below my shoulders.
I dated my bottle of Kirkland Signature shampoo when I opened it on November 7, 2012. Today is July 7, 2013. So, a bottle of Costco shampoo should last you 8 months.
The secret is to use just the amount you need. I am not saying you should use less than you need. I use plenty amount of shampoo. I just don’t use more than I need.