Probably 20 for me...I've done much longer rides in regular clothing, even jeans and running shoes, but dressing has become part of the ritual.
FWIW, I wear mountain bike shorts and ordinary T shirts for nearly all my riding, including centuries. I have three or four pairs of Lycra shorts and several jerseys, but haven't had them out of the drawer in years. When I say "dressing for a ride," I mean baggy MB shorts and a T.