How much could spice extend a human life in Dune?

In the Dune series, spice is noted for its geriatric properties - the ability to extend human life well beyond the natural norm. In "God Emperor of Dune", Leto II reflects on that property:

Without the geriatric properties of melange, people live and die according to the ancient measure-no more than a hundred years or so.

However, most of the humans we see in the series are of an age well within that norm. Obvious exceptions would be Leto himself or the Duncan gholas. However, that begs the question of what lifespans actually were in a spice-rich civilization.

How much could spice extend a human life in Dune, and what character is an example of it?

Note that by an example I mean someone who lived beyond the "hundred years or so", yet still maintained youth and/or vitality.

Answer

There's a quote in Children of Dune that seems to address this. With the regular ingestion of melange, the rich can expect to live several hundreds of years (e.g. 80-100 years x 3).

Without melange and its amplification of the human immunogenic system, life expectancy for the very rich degenerated by a factor of at least four. Even the vast middle class of the Imperium ate diluted melange in small sprinklings with at least one meal a day.

Heretics of Dune suggests that someone with extensive access to Melange and general good health could live in to their 300s

He [Teg] was, she knew, four SY short of three hundred. Granting that the Standard Year was some twenty hours less than the so-called primitive year, it was still an impressive age with experiences in Bene Gesserit service that demanded that she respect him.

and

Was he [The God Emperor] driven by the desire for long life? He lived more than ten times the normal span of three hundred SY, but consider the price he paid.

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Those who aren't wealthy (but merely middle-class) also experience a lesser effect and would, presumably, expect to live into their hundreds.