21 minutes an hour was no big deal in one small market I worked. We were on the Mexican border. The Peso had been tanking and leveled off somewhat before becoming 1 old Peso being equal to 1 new Peso. The #1 station across the river (La Rancherita del Aire then at 580 only with a huge signal and following) was charging about $1 US per spot. One station was selling 1,000 a spots a month for $5. Most stations across the border were charging about 15 to 25 cents US per thirty. The bottom of the barrel station had about 24 to 36 spots an hour at about 15 cents a pop. As a result, we were lucky to get our $2 a spot with a free play after 6pm for every one you bought. A typical package was $400. These were thirties. We had lots of 60 second agency spots mostly for soft drinks and beers and maybe a McDonalds or such. I'd say the average was around 24 minutes an hour based on a 24 hour day...about 48 to 52 units an hour during the day.
We averaged about 7 to 8 songs an hour. We had 5 minutes of network news hourly. Generally we ran 6 spots between everything, usually 3.5 to 4 minutes per break (ie: 1 or 2 at 60 seconds and 4 or 5 at 30 seconds). To fit it all in we had to run a break going in to network news or coming out, sometimes both. I literally threw the hot clock out the window and went all requests. One day, an election day locally, I started the 8 am hour with a song. After that first song, I played constant commercials until :53 after the hour breaking every 3 spots for a Time and temperature, quick weather forecast or a live PSA to break it up. Our log, then typed on legal pages, was single spaced and 2 pages per hour (back when the FCC required such). By about 11 pm we could go with 2 in a row most of the time. I recall I had 42 currents and about 126-128 recent gold (recurrents and up to 5 years old) in the studio. Some were dayparted only for after 6 pm.
Our saving grace was we were the only English language station on the dial and the stations in Mexico, of which one was American Contemporary Hits, all ran more spots between songs...as many as 15 between a song by my count but mostly in the 8 to 10 range.
After a 6 hour shift you were mentally fried trying to separate competitors, answering phones and back timing to network news and such. That's not to mention only one of our 3 cart machines would actually cue up and we had so many spots sometimes a 30 was on a 2.5 or 3.5 minute cart because we had run out of 40 and 70 second carts. If the cart was upside down on top of the cart, it meant it still had to be cued up in the good cart machine (we had a single play robbed from an automation system, an old level engage cart machine and one only about a year old). We played records then.
You'd think nobody would listen but you'd hear us everywhere and we likely got about 70 to 200 calls an hour (the 200 pretty much in the evening). At night, lots of kids would stop by to request songs as they cruised town.
Actually for a small market station it was not as horrible as you might think. We had a Tanner CHR jingle package with a couple of the ever popular (at that time) shotgun jingles and a meteorologist doing our forecasts (Pat Pagano). We had a fulltime news director too. This was the early 1980s. For our lack of experience, we sounded pretty good since we all came from large cities that influenced us and we had a heck of a lot of fun with lots of interaction with listeners. For example, I let people jock a song (intro it by phone) then let the audience vote a winner each day. All the week's daily winners went for a final vote on Friday, then got to go on the air with me Saturday night for an hour and even pick the songs, saying hello to anyone they wanted.
By the way, on the road a couple of weeks ago, I was passing through Pecos, Texas and listened to the AM there. They started the 9 am hour with a solid 20 minutes of commercials. I couldn't believe it. The oil field traffic made it seem more like driving in Houston and I understand Pecos is the 2nd fastest growing small city in the USA. Granted most of the spots were aimed at the oil field worker with quite a few spots detailing employment opportunities.