My first dd self-weaned around 4.5. She nursed at least once every day of her life for over 3 years (including during my pg with DD2 and immediately after delivery -- we nursed in the visiting room at the hospital!). I struggled to put limits on her because at first she wanted to nurse every time I nursed DD2, but she did get over that in a few months. (I would offer to read stories, play, etc., but she really wanted the boob!)
Somewhere around 3.5 she started lowering the intensity and only wanting it once or twice a day (bedtime, for one biggie). I also limited her time and offered distractions, but never cut her off completely. By age 4 she would even skip a day here or there especially if we were out of our routine. Around 4.5 she pretty much stopped, although she still asked once a week or so for a while. I still remember her asking, my saying yes, and her looking up with confusion, "I think I've forgotten how! Giggle giggle..." and that's how we knew we were finally done.
DD1 (Oct 99), DD2 (Sep 02), DD3 (Oct 09)