How long did you breastfeed for?

My son is 9 months and has been exclusively breastfed. I plan on going as long as possible. My dear MIL dosent agree. She didnt want me to breastfeed from the start because “it wont fill up the baby” and its gross etc. Anyhows now shes saying i have to stop breastfeeding once my son is one because my milk will go bad. I know thats not true. How can I politely tell her that i dont need her input (: . She always says i dont know what im doing because its my first kid, but i do my research and you make milk for a reason. She dosent get that and im getting fed up because she never listens.