this is the answer i found on Philips Avent website:
http://www.p4c.philips.com/cgi-b ... 50741_63.150.131.87
Question
I read on the VIA instructions leaflet that the Cups are only suitable for 4 uses, why is that? Are the cups deteriorating in any harmful way for my baby?
Answer
Not at all. We recommend an average use of 4 times because the Cups will become more flexible and we cannot guarantee they will get properly sealed with the lid anymore and you might experience some leaking.
尋日問左吉吉島職員都係話可以用5次!番到屋企睇佢入面書仔就冇講。唔好意思我唔識po張相上黎,本書仔淨係話"we recommend using via cups several times and to discard when showing signs of wear and tear"同埋"before each use, inspect the product. Throw away at the first sign of damage or weakness"