new bottle nipples?

Leah
My baby is breastfed but attends daycare during the week and gets his milk from a bottle. I have two questions:
1. How often do you replace the bottle nipples?
2. He is 6 months and still using a level 1 nipple. Should I change that? My concern is making it too easy to get the milk out and then he no longer wants the breast. 
Any advice is appreciated!