I would say when LO is getting frustrated and tired out and fussy during feeds, and the guidelines as to when to change teats help. My LO gets fussy on the bottle and keeps turning away as if he can't be bothered to drink it if it won't come out fast enough lol. I have Dr Brown bottles and posted a similar thread the other week, we have now changed to stage 3 my LO is 5 months on monday x