We went for the graco 360 from Smyths and it last roughly to 4 years… it’s a good seat! Highly recommended We upgraded at around 6 months as she would scream so much when in the car seat. With this chair its problem solved
When the current one is outgrown by weight or height. Or when you want a change
We swapped from our infant carrier to our convertible car seat at around 9 months, we had a Cybex cloud z and the weight limit is 13kg so he is still fitting in it now even at 15 months x
It depends on the seat you have and when that is outgrown. Most rear facing seats you won’t get a good fit until baby is around 6-9 months depending on their size.