How long did you breastfeed your baby? why?

I taught my breastfeeding class this past week. It's one of my favorites to teach.  My students know that I have 3 kids, so I always get asked the same question at the end of the class, "How long did you nurse your babies?"  I tell the truth, 15 months, 16 months and 18 months.  I can always tell that they are surprised.

So, I'm curious, how long did you nurse?  Did you feel pressured to wean?  Were you uncomfortable nursing in public after a certain age?  How long should a mother nurse her baby?  How long is too long?  Please share your comments.