How long is it best to breast feed your baby?

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Nowadays, hospitals are encouraging mothers to breast feed. My mother told me that she breast feed me until i was 3. As for my first daughter, my wife only breast feed her for 1 months and had to stop on the 2nd month due to work. For my second daughter, she is now 1 months++ and is still breast feeding. How long is it best to breastfeed? Thanks