This is mainly an answer to a different question — ”When did the phrase switch from a literal to a figurative meaning?” — but by documenting it I do answer the question, so pause a second before downvoting me.
A Google Books ngram search for “mileage may vary” brings up a bimodal graph:

The first portion, starting about 1970, peaking about 1980, and bottoming out in 1990 almost completely consists of literal usage in terms of miles per gallon, e.g.:
“Your mileage may vary due to how and where you drive, truck's condition, and optional equipment.” Popular Science, June 1978
In the second, later, peak one finds the term used mainly in the figurative sense mentioned by the poster. The first linked page for the period 1998-2011 includes books on
bed bugs:
“And in dealing with animals, the phrase your mileage may vary must be considered”
childcare:
“As with anything else kid-related, your mileage may vary!”
web programming:
“But your mileage may vary. If you view a Web page…”
and real estate marketing:
“Your mileage may vary, but not by much. If you rely on yourself to send out those letters…”
Shows how useful Google Books ngram can be. But then again, your mileage…