This subject keeps getting brought up over and over again, in different places and in different ways. Here's a link to one:
The radio industry has asked the question, and the answer usually is none. People want music for free. They don't want to sit through commercials. When YouTube gives the option of clicking out of a preroll ad, the majority of people click out. That's just one commercial. So it's not a question you ask, because the answer is obvious. No one wants to listen to commercials. Period.
There are some who think if there are more frequent short breaks, it's better than two longer breaks. The research has been done, and the results are about the same. If a person is going to switch out of commercials, they'll do it at the first one, regardless of the length of the break. When PPM started 15 years ago, the research showed clearly that a music station is better doing two long breaks than frequent shorter breaks. That's why it's done.
News and talk is completely different from music. There, they do frequent long breaks. They typically will run more spots in news/talk than music. By a factor of about 40%. That's why news/talk makes so much money. I was listening to WFAN last night and one break had 20 spots.
The number of spots a station runs is based on the costs a station has and the price they get per spot. Take the costs, divide by the amount each spot sells for, and you get the number of spots. What makes it more complicated now is advertisers don't want to buy anything besides morning and afternoon drive. So the money you make then has to cover your costs for the entire day. The other problem is costs keep going up, while spot price remains pretty constant (thanks to competition). Spot price is set by the marketplace. When you have a gap between revenue and expenses, that's what leads to staff cuts.
Historically, the number of spots a music station runs per hour has been pretty consistent: About 12-14 minutes. I have program logs from over 30 years ago that look about the same as they are now. At one point in the early 2000s it got up to 16, but the reaction was negative, so they cut back. I'm sure a lot of posters will give examples of stations that did less. Some did, for various reasons. Some still do. Some do more, but typically it's only for one hour, and they're make-good spots that were missed. There will always be exceptions.