When should a mother stop breastfeeding?

So I've decided to breastfeed and I truly don't know much about it. I'm taking a class that my hospital provides in January so I can learn more but I was on Facebook a moment ago and this woman was breastfeeding her toddler on video. I have no problem with public breastfeeding at all and was interested in the video so I clicked on it and she was talking about how feeding her 2.5 year old is okay. When I started reading the comments SO many people were bashing her. I really didn't understand why it was a big deal honestly, but then I thought about my niece who is 2.5 years old and was formula fed. She's been weaned off that for A LONG TIME. She's till had milk before sleeping but that stopped early before two as well. My question is when should a woman wean her child off of breastfeeding? And is it really hard to wean them off?