Do you think its advisable to get a second hand sports catted downpipe?

Obviously decat exhaust bits are pretty safe buys as they come with lifetime guarentees so u know they're not gonna be defective if u get a second hand one. With a sports catted downpipe i assume at some point the cat will need replacing, so a bit worried about buying one second hand cos even though its only meant to have done so many miles, still concerned it may be knackered!

How long does a cat last for?