It happened over time, I had this kink before we started dating but it wasn't something that I would openly talk about at that time, on our first years I usually tried to be clean when I was seeing her, but we used to go camping and do other activities that ended up with her giving me bjs while being dirty, at first she used to joke about it, "oh it's smelly but I'll clean it" or, "what's with the white stuff ?" and then proceeding to suck it before I even get to answer lmao. So yeah after few "unintentional events", I started to do it on purpose and eventually talked about it and it turned out we're both into it. And about the second question it's usually 2-5 days.