In my experience, a lot. For all practical purposes, every couple I've been 1/2 of has been a mainstream couple--the woman has never been an avowed fetishist, we've never discussed tickling in a sexual context, and it's never been a fundamental part of our erotic relationships. And yet, with the rarest of exceptions (seriously. Like, maybe 2?) every relationship I've been in, including the current one, has involved the woman tickling the living crap outta me on a regular or semi-regular basis.