About an average of four or five per year-
the '95 Civic gets about two;
the '77 Chev 454 pickup about one or less;
the Honda Harmony II lawn mower gets changed once, sometimes twice, yearly;
my Karcher pressure washer with Honda engine, once a year or less;
and, the Honda multi-gizmo with an itty-bitty four-stroke cycle engine gets changed about once every two to three years or so.