I imagine the process is slower or perhaps it starts deteriorating when their spirit is completely disconnected from their body,as for Solas he's a mystery since we don't know whether he's Fen'harel embodied or just a similar case to Flemeth's.
I also suspect that Abelas and the rest of the sentinels are rare cases as well as their bound to Mythal,which might prevent them from being entirely disconnected from their bodies.That's my theory for now.
Well, according to Felassan, they had people to tend them and give them special herb concoctions, until they eventually learned to draw energy directly from the Fade. I suppose sometime after that is when they begin to deteriorate? Either that, or they only deteriorate if they never learn to draw from the Fade. Still, you certainly can't live without food and water, no matter how in stasis you are. I guess that's why uthenera is effectively the ancient elves' way of choosing to die. It seems like a terribly sad way to go, and yet I suppose it must at least be painless?
I'm not sure if it's uthenera or something similar that the Sentinel Elves are in, since they are effectively sleeping and waking to obey Mythal's will--defending the Well. Uthenera seems more like something that you stay in until you're ready to leave or you die.