That is a very personal decision...
I breastfed all of my 3 older kids for varying lengths of time. The longest I nursed for was about 2 & a half years. My daughter was an early talker and would ask for it which was weird, but I knew it wouldn't last forever. I would only let her nurse at bedtime by the time she was 2.
My baby girl has Williams syndrome and she is nursed exclusively. I would probably consider nursing her til she is 3 due to the typical developmental delay associated with her condition.
I think this is a personal decision. I can only say that the benefits are unlimited for mom and baby and nobody should be judged for doing what is best for herself and her family ( :