I voted 19-24 months, but that's really not true. I have three kids and was thinking I could only vote once. It was 18 months with ODS - and I cut him down in order to get my fertility back. It was 16 months with DD and I was tandeming - but didn't have to cut her down. It was 25 months with YDS while tandeming. So, I voted for the average which put it in 19-24 months even though none of my three were exactly in that range.
As for what was going on:
*ODS - I cut him down from 20 times at a year to 12 times at 18 months to 5+ times at 24 months. Though I did ovulate before that first period at 18 months, my luteal phase was too short for two cycles. That's also the time we had the most drastic decrease, from about 12 to 6 times.
*DD - She nursed 8 times in 24 hours at a year. She cut herself down to 5 times at 16 months. I was tandeming, but I can't remember how much that added in. When she was 16 months, she nursed just to nap for daytime. Then it was before bed, two night wakings, and first thing in the morning. So she did have long periods of time without nursing. I did ovulate before it returned at 16 months, but again, my LP was too short for two cycles.
*YDS - He was nursing excessively and not cutting down at all when it returned at 25 months. I was tandeming, but DD was down to 1-2 times. I am certain I didn't ovulate before. Actually, it was very rare for me to ovulate for the entire next year and then some. He cut down to getting at least half his calories from solids a couple months after 3 years old. That's when I started to ovulate regularly again.