Have the required fetish level, and then just keep going in and out of your room in the inn.
I have ass fetish lvl 10, I was in and out of the room for about 40 minutes interspersing the days and nothing :c
its either level 25 or 50, every 25 levels is a new "Tier" of the fetish, pretty sure that anything outside of that makes no diference even in combat
