I don't really qualify since we were preventing and then trying, but my first fertile ovulation was when my daughter was about 25.5 months old and then conceived the next cycle with ovulation when she was a few days shy of 27 months. I can't say what it would have been if we were NTNP, but it would have been very unlikely before 25.5 months, probably pretty likely then if we hadn't been preventing because I remember it taking a lot of self control to abstain then, and certainly possible at 27 months if timing was good. My children's conceptions were 6 days less than 3 years apart and would have been just over 34 months apart if I had conceived one cycle earlier.
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I'm not sure what you count as not preventing, though. We used ecological breastfeeding as a mothering style, knowing that it also provides a natural child spacing effect. I'm a little bit of an outlier for an American woman using ecological breastfeeding. Most women using ecological breastfeeding will experience 1.5-2.5 year spacing.