The effect of the Great Rings upon Men

[Inziladun]

Originally Posted by Mithadan

To me, the implication is that the Great Rings eventually turn a Man into a naked spirit. The hroä (body) becomes separated from the feä and the body dies leaving only the spirit which, through the power of the Rings, can still manipulate and be partially present in the physical world. Gollum and Bilbo possessed the Ring for a long time. They were partway through this "conversion". So perhaps it is not a question of their life span being extended, but rather that they had undergone a fundamental change in their nature.

The spirits even of the Nazgûl though were apparently still connected with physical bodies, and those bodies did occupy the world of the Seen as well as the Unseen. If that was not so, horses could not support them, nor could the "disguises" in the form of cloaks and hoods they wore in the Shire, have covered them.
As long as the Ringwraiths' spirits were subjugated to Sauron's, they had basically become extensions of his own fea, and thus would endure unless he reached a point he was too weak to maintain his hold.

That seems to make sense to me, anyway.