The essential resource on this topic is Straight From the Heart by Tehilla Abramov. Mothering Your Nursing Toddler (La Leche League) also has some references to Talmudic material.

I will post the specific halakhic citations at a later date, but the bottom line is this:

Halakhic sources unequivocally expect nursing to last for at least two years. In fact, a woman's status as a "nursing mother" in the halakhah lasts for two years, whether or not she continues to nurse for that duration. According to later medieval codes, a boy can be nursed until age 4 if he is healthy, or age 5 if he is "weak." A girl can be nursed as long as desired. (I expect that the difference had to do with rabbinical discomfort with boys' continuing proximity to mothers' breasts.)

If you need more specific sources in the meantime, please email ask@journeymama.net.