Well, Fly and my "rule" is after having sex 3 times with new person, on the idea that if it's gone that far, it's likely this person will be around for a while in whatever role (FWB, fuckbuddy, love) and may impact life and family stuff.
In reality, I tend to want to talk about people right away, as soon as I know I'm interested in them, because I'm a talker.
Fly is not, and pretty much sticks to the 3-times guideline. However, he's more into new and shiny, and leans toward the one or two-night-stands rather than ongoing things, and even his ongoing lovers are pretty strictly friends (in a real sense) with benefits and not romantic entanglements. So, we rarely discuss his partners, and they rarely appear in day-to-day life except as far as negotiating child/chicken care.
Moonlight and I don't really have anything clear about this, and it hasn't come up because I haven't been seeing anyone but Punk and Fly since i met her. I will probably tell her right away also, and it will probably be ugly because I'm pretty sure she's monogamous at heart. But I respect and love her, and in my personal code that means I have to be not only honest, but also forthright with her.
Punk really couldn't care less.