instand1 - 6:29 am on May 8, 2013 (gmt 0)
Seems to be very difficult to give a good answer: It depends on the product or service you offer. In some markets you can get 1 order per 10 clicks, in others 1 per 200 clicks.
First thing to check: Is the bounce rate lower than 50%? If not: The landing page and the match between keyword, ad text and landing page should be optimized.