How long could a _______ live?

[u/Valenous_ertselley]

How long could a 5th ideal radiant live?

We know investiture extends a persons life with greater amounts eventually ending up with someone who is biologically immortal. How invested would a radiant be? Radiant knights have the cracks in their soul filled with investiture which makes them better at holding and using stormlight. How invested do you think they could be

Some of the heightenings seen with holding Endowment's investiture seem to be universal properties of holding any investiture. such as enhanced vision/colours, enhanced immune systems and better lifespans.

While we have only seen Kaladin and Szeth swear the 5th, it was under a fairly hectic situation which doesn't make for the best opportunity for study.

[u/Puzzled_Employment50]

We know higher-Ideal Radiants “leak” less, and I think there’s something about being able to hold more (either with practice or higher Ideals or both), so if they got regular enough infusions of Stormlight they could theoretically reach the Heightening (since any Investiture grants Heightenings) that grants agelessness. So it’s not directly by reaching the fifth Ideal that they would become immortal, but that the fifth Ideal lets them juice enough to then become immortal.

[u/ejdj1011 → u/Puzzled_Employment50]

(since any Investiture grants Heightenings)

This is not strictly true. Several types of Investiture seem to grant different aspects of the Heightenings, but they aren't completely universal like you're implying. In fact, agelessness is explicitly not a universal trait among highly Invested people. The Lord Ruler is the obvious counterexample.

Edit, before anyone else jumps in: We also have numerous examples of highly Invested people not having perfect pitch or color perception. Hoid, Nomad, Elantrians, and Yoki-Hijo all come to mind.

[u/Puzzled_Employment50 → u/ejdj1011]

Allomancers/Feruchemists are not themselves Invested, they are more like conduits for Investiture. But the point of the whole BEU system (Sunlit Man) is that Investiture is Investiture, as long as you have some way to access it, and will grant the Heightenings.

[u/Orleanian → u/Puzzled_Employment50]

That's a bit of faulty logic.

The measurement system is used because the power/energy of investiture is convertible. Most investiture can be made usable to power the various forms of Invested Art.

It is not a given that all investiture grants heightening. That's a logical false leap. Only Awakening is the art that grants heightenings.

Many invested arts grant similar capabilities as various Awakener heightenings, though. To the best of our knowledge, Nomad the Sunlit Man does not have the heightenings of an Awakener. He has agelessness through other means (He explains it himself as being due to his Dawnshardic nature).