My baby used the lowest level teat till he was 4 mo. Don't have to change unless u see a change in her feeding pattern or she starts to get fussy during feeds, or takes much longer time to drink. For me, at 4 mo my boy started to get fussy. He wld always finish his 160-180ml milk but at tt time he took so long to drink and only finished 60ml. When I upgraded the teat, his fussiness stopped. Some breastfed babies don't even change the teat level till a year old as teats shld mimic mom's milkflow.