I've asked this on another forum as well.
the responses i've have are:
Newborn & Small sizes (overall) are normally adequate for use on several babies as they only get used for a few months per child.
Sized Pockets/AIOs which are sized will depend on how they're cared for obviously those with the pul covers can deteriorate with tumble drying but as they're sized the same as above applies.
BTP Pockets/AIO do get alot of usage so are more likely to have the PUL and Elastic deteriorate and may need replacing. (nappies can still be repaired)
Cloth only 2 part BTP nappies benefit from have the PUL cover separate so that can easily be replaced and they're less likely to be elasticated (which can also deteriorate) so the nappies themselves could potentially be used for a number of children but you may need to replace wraps (more so for the larger most used sizes)
it kind of makes sense, when you think about it.
they also stated that things like aplix and poppers can easily be replaced, also some WAHM's will fix them for you or replace elastic etc but i guess those with PUL covers can be more difficult i.e. BGs
Care Tips
- reduce tumble drying to a min
- take care when stuffing nappies (if PUL brittle i have read comments of hands going through the other end of the nappy)
but i will def save them for baby no 2 
and have an excuse to buy more 
