I can attest to no commercials for a good while when a station flipped formats back then. In Dallas/Fort Worth, KFWD 102.1 switched from beautiful music to top 40 and I suspect it was a good 6 months before the first commercial aired. Even some flips when there was a combo, the FM might not run a commercial for a month or so, or at best have perhaps a spot an hour at most. This was back in the last half of the 1960s thru at least about 1974 or 1975. Somewhere around 1977 KFJZ FM appeared as a Top 40 going commercial free (and announcing commercial free hour number xxx and then commercial free day number ??). As I recal they had a big contest for when the first commercial played.
In Kansas City about 1968, KXTR, an FM that seemingly aired only 'certain hours' ( i was a kid but I think evenings and then maybe 12 to 18 hours a day weekends) went 24/7 with Classical Noon to Midnight, Jazz overnights and Beautiful Music until noon. I knew a couple of the jocks. They knew I had a part 15 and let us grab all the AP ticker news stories from the trash can for that little part 15. I recall they got their first big advertiser about month 3 and within a year they were bragging they have about 20 commercials a day. They weren't cheap: open rate $25 in 1969!
The first commercial-free launch I ever heard was KFMB-FM, San Diego's B-100 launch in March of 1975---100 hours, commercial free. Prior to that, I can't think of one (though I certainly hadn't heard everything). I know some programmers had a philosophy that, even on day one, you wanted to sound big and established...though that approach fell out of favor as the 70s wore on.