When to change Dyneema/Spectra stays?

[Norm van't Hoff]

Hi all.

How do I decide when to replace, Dyneema stays? The boat is 12years old with (I have to assume) the original rigging. I can't find anything definitive online, they're obviously vitally important, but it's an expensive decision, can anyone help?  

Many Thanks, 

Norm

'Jumpa Lagi', Tiki 38