You specify you want it at the news times in the morning, random spots throughout the day and then before the news again at 5pm. Not on weekends, not after 11pm or before 6am unless they are free.
You guys that fall for the bartering for Airtime trick are fools. I did work for Beasley broadcasting for years. (2nd largest radio station owner in the US just behind Clear Channel) and let me tell you. They have cash by the wheelbarrows full. You can get paid cash if you want to get paid. The station managers always try to barter because, they pocket the cash they didn't pay you.
For example if they take your $5000 invoice and give you Airtime during say drive time. Its because they lost the Auto Dealer for that spot and its dead air. They put it down that they sold all the spots for $1000 for the month, and submit a request for reimbursement for the $4000. To the corporate office for the van wrap, they pocket the cash and you got screwed.
Airtime is dirt cheap if you buy it on your own. I used to buy up stations time slots all the time. They like to have them booked up 3 months in advance and if they aren't booked up you can usually buy em up for pennies on the dollar of value. That is what all the big marketing firms do. They call around all year buying up air time to resell em. No reason if you got some cash handy you can't do the same thing for yourself and sell some of the spots to your customers or reward loyal customers. Plenty of voice over companies will put together a :30 second spot for $500 and you can use it for years.