I think the worst thing that happens to the nappies is that the elastic can break in them. This process is sped up by using the tumble-dryer alot, putting elasticated nappies straight onto the radiator and hot washes (if you dont mind washing at 40 degrees, the elastic may last longer!) Poppers and aplix can also be replaced if necessary.
There are nappies that have elastic that can be replaced easily by yourself, but other are a bit more of PITA
I've had some WAHM nappies where the elastic has gone in them, or is starting to, even after not that much use, but when it happened with one of my WN's, Suzanne kindly offered to fix it for free! So it should last ages again
Many WAHM nappies are superbly made though, and will last for ages! 3 or 4 of my BGs have dodgy elastic now and its annoying as i'd like them to have got to a second baby before that, and on a couple of them the aplix isn't very sticky anymore either. Again, aplix can be replaced with a little effort! I have 3 Flexitots, that having been used just several times each, have holes in them
My size one nappies are going to be used on a second baby when SILs baby is born (for some of them it will be a third), and if theres any life left in them then I'll use them for my next one as well! Small nappies tend to do more babies as they're not used for as long as bigger nappies.
Flat nappies, inserts and boosters will last for years unless they have been badly stitched, and in theory shouldn't really stop being usable until they literally fall apart! My mum gave me the terries she used on me and my sister and while they're a bit thin now, they are very usable. PUL wraps may not last as long - they tend to get leaky with a lot of use and I'm not sure if that many of them make it to a second baby, but it really just depends how much they've been used!
Id imagine some of the commercial brands like motherease and tots bots should definetly last until a second baby. By that time, you may have made your savings already and really just be cloth nappying the next baby for free!
Most WAHM nappies can be repaired by the WAHM again, even if they charge a small fee then the nappy is usually good as new!
When I think of the fortune I have spent on my nappies, I like to think that they will do the next baby as well!