With my first son, I think I started when I stopped pumping during the day, which was around 14 months. I still nursed him at bedtime and on the weekend, but he also stopped the night time nursing a little before that time. It was enough to stop the hormone from flowing that stops you from ovulating. You aren't ovulating at this point since you don't have a period, but obviously the first month you have a period you will ovulate and probably not know when. I know my period was pretty weird even for the first 3-4 months...I would have like two weeks on, two weeks off, and the bleeding was spotty and not as heavy. My Dr gave me a hormone pill of some sort to take during my next cycle that "cleaned me out" and that got me going back more like normal. Def recommend reading Taking Charge of your Fertility and learning to chart as it shows you in the book, although charting even is hard until you get back on a regular schedule. You can also do the charting online at fertilityfriend.com. With my second I had an IUD put in when he was about a year, and he just weaned at 2 1/2 and the IUD pretty much stopped any semblance of a period I was having, so I can't speak to much to what happened with that one.