Offhand, I don’t think I came across a specific Tipitaka reference on the lifetime of a pure octad (suddhāṭṭhaka).
However, any rupa has a finite lifetime. The solid Earth lasts for a fourth of a Maha Kappa, so the lifetimes of MOST OF suddhāṭṭhaka should be in that time range.
See, “The Origin of Matter – Suddhāṭṭhaka”
– Also, see, “Buddhism and Evolution – Aggañña Sutta (DN 27)”
I hope you read specifically post, “Does any Object (Rupa) Last only 17 Thought Moments?”
You probably those above cited posts, but I just wanted to make sure.
Finally, certain suddhāṭṭhaka may have longer lifetimes. For example, the lifetimes of arupavacara Brahmas are MANY Maha Kappas. Thus, those suddhāṭṭhaka associated with their hadaya vatthu would have very long lifetimes.
– However, there is no rupa that will last forever. All sankhata have finite lifetimes.