For many extended breastfeeders, when the child self-weans, which is normally between two and a half and four (sometimes earlier if the mother is pregnant again). That's how it naturally happens in cultures all over the world, if there aren't any strange hang-ups about breasts.
I ask again, why is it "not strange" to raise cows, take the baby cows away from the mothers, milk them, transport the milk in big container lorries, process and bottle it, distribute to shops, go out, buy it, take it home and store it in the fridge for when a toddler wants a drink, but "strange" for the toddler to drink the milk that its mother is naturally producing for it? We all get that you, personally, are weirded out by it, but do you have any rational basis at all for your attitude?
There are health benefits to breastfeeding. There are health benefits to a healthy diet. But those benefits are cumulative -- there's no "OK, my child is as healthy as he can possibly get now, so I shan't bother doing anything else to watch out for his health". You have a perfect right to not breastfeed at all, or stop at 3 months, or stop at 6 months, or stop at a year, or continue as long as your child wants. Just as you also have a perfect right to feed them Fruit Shoots or chips or chocolate or organic hand-knitted tofu muesli.