Actually you get 90 days every 6 months, however you divide it.

Funny thing though. I've made 18 trips across the Atlantic in the last 2 years and I've never seen a customs agent check my passport in a way to determine if I'm over my stay. In fact, technically, I'm over right now because my Visa has expired, but I've come and gone twice this summer with no one raising an eyebrow.