I don't really have a single source for the 5 - 7 years, I've read it before and kept it in my mind, the article I linked was for the shrinking brand loyalty as I thought that might need more confirmation, that said

https://www.nasdaq.com/articles/heres-how-often-americans-re...

which implies less than 5 years for 2/3s of Americans, although not sure what the average is.

I seem to remember reading somewhere that most people will own 10 cars in their lifetime but querying How many cars does the average person own in their lifetime gives me 8.

Assuming car buying age is from 18 to 72, that gives 54 years of car buying, and 8 gives us 6.75 years per car if the average owns 8 cars. 5.4 years if the average owns 10 cars.