It also comes down to the market your are selling to and your products. For example low cost products are going to sell faster, but make you less money. whereas high value products are going to take a lot longer to sell but make you a lot more per sale.
My rule of thumb is... high value products will have a much lower sale/click ratio. so you will get a lot more people simply clicking through browsing your pages and leaving.
You just need to pay close attention to your ROI.
If you are new to adwords you might want to consult with a third party company to manage your campaign. There quite a lot of god companies out there who specialize in this field... Hope this is of some help.
Mack.