Did they fail it on being too slack (so it just needs adjusting) or on being too worn/stretched ( so it needs renewal) ?
If it's the latter then they must have said how they measured this.
Generally I believe a chain to be shot when one or more of the following:-
1. Chain can be pulled free of the rear sprocket far enough to be obvious/see a gap.
2. Chain needs more and more regular tensioning.
3. Rear sprocket is worn/hooked. There is no point just renewing the sprocket - the new one will soon be mangled by the old chain.
4. O rings are damaged/missing
Chain life can vary immensely. An ill adjusted dirty or dry chain can be destroyed in no time especially with heavy handed throtle control / wheelies etc
A chain kept well lubed (Scottoiler etc) will require little adjustment and last for many thousands of miles.
O rings are your friends. Look after them and they will look after your chain.