how long should the nipples be used before discarding them,,?
How long do the nipples last once opened and sterilized many times over?
It is recommended that you check the nipples on your bottles and pacifiers every time you handle them. Things to watch for which would indicate they need to be replaced are discoloration, thinning, stickiness or swelling, cracks, tears, or breaks.
Ditto the answer by BE, I check them while hand washing the nipples by letting water fill them and with my thumb push them from the bottom to look for holes from bitten nipples. Otherwise, I have used some for two years.