I don't have my own experience to share, but my SIL got pregnant with her second son while nursing her first. She said that she was told her milk would decrease, but it didn't. She and my brother wanted to just get pregnant with #2 asap after having #1. It took them a few months, and my nephews are 16 months apart, I believe. It might be closer to 17; to be honest I'm not remembering my new nephew's birthday (I've been busy with moving since before he was born.) I know she nursed her older son until just a bit after he was 12 months. She didn't have to wean, she chose to, but I know she nursed for several months while she was pregnant.