I have had 2 people tell me it is effective, that is until your periods return. However, you ovulate before your periods return and I happened to notice a huuuuge lot of ewcm before I got my period back, so I'm pretty sure I would have been fertile.
I wouldn't personally rely on unless you are open to having another baby.
It doesn't work, I have proof although she was sleeping thru, so wasn't doing night feeds, but still thought I'd be covered