A very interesting topic to say the least. Having put 45 years in radio, small market, I've worked at stations that were VERY HEAVY on commercials and some that were very sparse. One old P.D. of mine told me, that you should NEVER go more than 2 minutes without a change of pace, i.e. a quick talk segment, live show promo, song teaser, weather, etc. I tend to agree with this as I feel myself tuning out the commercial set after about the third one. Also along the way, some stations, set a maximum limit on number of minutes per hour (after the FCC quite the regulation) and if they were sold out then they simply raised the rates. I have also seen a sstem where the long term contract advertisers were place first on the logs (sets) as sort of a reward for thier contract.
As an advertiser for my own business I always tried to by 60's as they tend to run first in the commercial set. I always tried to keep my commercials fresh, and not beat a comedy spot into the ground. For my tastes radio has changed into a automated juke box with nothign to attract the listener. Ok, that is a generalization, but I go back to something my first P.D. told me, "be informative, be entertaining, or BE QUIET." I belive that applies not only to jocks but to the station in general.