don't tell me how to breastfeed !

Erin • Mama to 3 beautiful girls
My husband just told me that his mom him yesterday that I need to be feeding our daughter for 20 minutes at each breast. 
This comes from a woman who has never breastfed a baby in her life and I guess HER mother in law told her that. (Not my husbands grandmother) 
Our daughter is 3 months old and never eats for that long. Sometimes she will want to eat every hour, sometimes she will go long stretches without nursing. 
How long does your 3 month old nurse for?