I agree with JAAs & Linkys answer for the most part. In our area the few companies who have advertised seem to do so during the 530am local newscast and then again at the 530pm local newscast. I have been discussing the benefits with the local station here about exchanging services as they have highly recommend their desire to do so. It almost seems that this is how they choose their contractors. The commercial length in qussetion airs for 60 seconds.
The other alternative that you could possibly check into would be a Radio Format that is listened to by 95% of the local businesses. This way you know that at some point throughout the day the market that you are looking to target will get your name. I am currently checking into this as well. Have an inside source. I will get back to you as to whether or not they would consider swaping services too.
Hope this helps.