Firstly, depends on how you are maintaining them. Milk has an oily feel to it so you need to wash and sterilise them frequently. I suppose your question means to replace? If so, I did it at different stages. Every 3 months till 6 month old. Then I look for signs of wear and tear. Else, 6mths thereafter. This is just for bottles. For teats, there are different holes for faster drinking /slower. That one strictly 3 mths.
jess