Q:

How to know whether it is the time to change the nipples?

How to know whether the bottle nipple is damaged or not? What are the indications and how frequently should it be replaced?

A:

Most brands recommend replacing the nipples on a regular basis usually 3 months I think, however I've found they tend to last longer than their recommendations. Usually by the time the baby is ready to move to the next flow the nipples are still working fine. It helps to take good care of them and not use brushes to clean them. 
As to how to tell if there's damage the best thing to do is compare them to a new, unused one. If there are marks, cuts, or big creases that is a sign that the nipple's pretty worn. Put some hot water in the bottle then turn it over and see how quickly the water comes out compared to the new one. If you have much more water coming out of the used one than the new one then there may be a tear or it's just plain worn. 

