lgn1 - 2:33 am on May 5, 2012 (gmt 0)
Once your adwords costs are 5x your profit without a sale you are 95% sure its a failure.
Once your adwords costs are 6x your profit without a sale you are 99% sure its a failure.
Stop advertising, and wait 30 days. If you don't have at least 2-3 sales. It's a failure.
You can try match type, geographic areas, negative keywords, etc, etc. Adwords have more layers of complexity than an onion.
But you are starting from a pretty deep hole.