Pre-Drake format on Top 40, it was 18 minutes an hour:
Bill Drake's commerical limit for KHJ was 14 minutes an hour, with no break having more than three elements or lasting more than 70 seconds.
So, you could have a :60 and a :10, two :30s and a :10, or three :10s, but not two :60s, three :30s or seven :10s.
It was easy for Drake to implement that at KHJ in 1965, because the station wasn't doing well in the ratings as an MOR and when he flipped it to Top 40, a lot of sponsors left.
Drake's genius was including in his contract that commercial limits were an element of programming and that he, not the GM or Sales Manager, would have control. When the ratings went up, increasing demand, the commercial limit remained the same and it was the spot rates that increased.
Drake had that clause in his deal for all the RKO stations that followed (KFRC, WRKO, CKLW, WHBQ).
The hourly commercial limit held firm for Drake's tenure at RKO, which ended in May of 1973. The only change was the length of the spot breaks, which went two two minutes and no more than three elements in late 1971. Drake said the change was to accommodate increasing song length by reducing the number of breaks per hour to seven from a potential 12 previously.