I just change them when they look like they are getting the normal wear and tear. E.g. colour change, holes in the teat

Great question, thanx for asking. I’ve had difficulties in the past wondering whether or not to throw or keep using. :-/

I didn’t replace mine until they started to wear and look damaged :/ I’m pretty sure there are recommendations on the back of teat packets, you could try reading them and see what they suggest

if they change colour or get damaged

Some go sticky when they need replacing. They change colour too. Once baby starts getting his/her first teeth you will find the teats start to get tiny little holes in them then gradually split. As time goes by the teats don’t last as long as teeth go through them. Some babies actually chew on them either while they are drinking or at the end of their bottle before you take it away. If they drop their bottle at all the milk will run out a lot quicker too.