As long as possible! I think AAP recommendation is at least one year, and that some may go up to three. I've been in countries where I've seen kids older than that nursing, but I know that here in the USA it can cause some tilted heads and comments.
I have two sons and had totally different breastfeeding experiences with both of them. One went two and a half years, got his teeth late, never was an adventurous eater, and has never been sick. He's twelve now. The other one got his teeth early, was a biter, (!!!), wanted to try every food he saw us eating and weaned himself at about eleven months. He is an adventurous eater and has been sick once. He is now almost two.