Just wondering How long would you guys say is the most time you would run a banner advertisement?

I have had an ad on my one site for 30 days the ad has been clicked 160 times. However NO sales as of yet When a person buys my end is $40. The banner ad has been seen by 10,000+ UV

Would you switch the banner ad to a different banner ad from the same company, or find a different company?

What formula do you use in testing your ads?

What do you feel is a good rate of return?

Thanks in advance