New poster here... To me, once a week was liveable. But when it gradually died down to once a month (getting shot down when I initiated got old), that was the beginning of the end. Sex isn't the ONLY reason to be married, but the intimacy is definitely one of the reasons I didn't just get a room-mate to share the expenses.
Is your wife on birth control? One of the ways that seems to be the most effective is the reduction in a woman's libido...
C