Breastfeeding Support Group
While breastfeeding is recommended by many doctors, it is not as simple as it seems. This is a community dedicated to parents who are choosing to breastfeed, providing a safe place to discuss everything from pumping, to questions of whether baby is getting enough, weaning, breast care, scheduling, supply, and more.
Be sure to wash between each use and sterilize at least once a day to prevent bacteria growth. I think you can probably use one for a few months (hopefully you won't need to) if you see any cracks or the plastic starts to look cloudy it's time to change.
http://www.nbci.ca/index.php?option=com_content&view=article&id=74:nipple-shields&catid=5:information&Itemid=17