I just listed some infant chinese prefolds (green edge). They are in great shape. DS1 wore them for 5 months, DD wore them for 13 months, DS2 wore them on-and-off for 3 months. Even if I don't count DS2 since he was in sposies, too, that's 18 months of use being washed every 3rd day. No holes, no tears.

Some of my regular sized prefolds are rags and some are fine. DS1 wore them for 2 years, DD wore them for 1.5 years, so 3.5 years total. They're only 6 layers, not 8, so thinner to start. They still lasted quite a while, IMO.

So, yeah, I think 18 months is too short.