I used to get the question, "When do you plan on finishing the whole breastfeeding thing?" My response was always, "I'm not sure."
I think it depends on when mom is done and when baby is done. Some babies decide that they are done with breastfeeding as soon as they get comfortable with a sippy cup. They are just too much on the move to stay still to breastfeed. I've heard various reasons moms choose to stop. One mom I knew wanted to start taking meds again for her own migraines. She couldn't do that, while breastfeeding. Another mom I knew just felt done. All the above are good reasons to wean from breastfeeding.
The important thing is that you and your child need to decide when to end breastfeeding. It's not for anyone else to decide.
Here's a Infographic for The Breastfed Toddler. put on by The Alpha Parent.