In the late seventies and early eighties, lower rotations were also common on the syndicated automated formats that ran on a lot of the smaller market FM stations. I recall Drake-Chenault's XT40 format running their powers at about 50x a week (at least on KSXT-FM in Walla Walla, WA), TM Stereo Rock at about 34x a week (KHQ-FM Spokane, KNWR Bellingham), and Century 21's Z Format seemingly similar in rotations to Stereo Rock (KFFM Yakima, which is still Top 40 today). But repetition was typically much higher in large markets where the stations were live and locally programmed.
I'd say that even after all of those automated formats were long gone, smaller market Top 40 stations tended towards less repetition that their major market counterparts. Through much of the 90s, R&R showed stations like KHKS in Dallas spinning their powers 99x a week, while at (for example) WKRZ Wilkes Barre is was half that.
So far as I'm aware, no Top 40 station in the U.S. regardless of market size rotates their powers as infrequently as WKRZ did back then.