My boy is 9 months and still on 1 😂 he's in the 80th percentile so he's definitely eating enough and finishes his bottle pretty quickly
It depends on the baby. My first was a hungry baby and got frustrated on slow teats so was on a fast teat by 4 months my second has only just moved up a teat size. My friend's baby never changed sizes and was content on the same size teat until they gave up bottles.
My son was EBF for 4 months then combi fed now EFF, we’ve had to change what teat he’s had as it got to a point where he would get bored because the flow wasn’t fast enough and he would just chew the teat x