Personal choice.
I plan on 12-18 months for lots of reasons. I would go to 2 years, but think beyond that I'd work on cutting back.
I went between 6-9 months with my first, and wish I had gone longer, I stopped because I was afraid she'd bite me (her teeth were coming in) and I was young (21).
Medically, there is still a lot of nutrition in breast milk even beyond 2 years. Many doctors and nutritionists friendly to breastfeeding note that breastmilk has all the necessary components for nutrition for the first full year. No solids are necessary to supplement. I think world wide the statistics are something like babies are 3-4 years breastfed. So I imagine developing countries etc. would have babies fed much longer than countries like the US where breastfeeding longer than one year is still sometimes taboo.
I'll see if I can find any articles for you.
