I wanted to feed my 2 oldest for about 6 months. I had trouble & didn't make it with either til past 6 weeks. When DS came along, I wasn't worried. I wanted to make my minumum 6 months, but thats all I thought about. If I hadn't fallen pregnant again he'd still probably be feeding. He was only a few weeks off 2 when I stopped him, coz the pregnancy hormones made it hurt too much to keep going.
Most people I know stop either due to work, health or pregnancies. I don't think I know anyone who set a limit & said thats it when the time came.