the caved-in stomach implies he sent himself into rudeus's house without any organs, right? or at least without the stomach organs. he has to still have his heart and lungs because without them he couldn't speak.
to get that sort of shape, he'd need to be without his liver, intestines, probably also kidneys, stomach.
if he just left his liver behind, he'd be unconscious within seconds, dead within minutes, because the liver attaches to major blood vessels. this didn't occur, so presumably he used magic to seal the blood vessels.
if you prevent blood loss, then the second thing that would kill you is the lack of pancreas. your brain will still burn through your blood sugar to keep itself functional, and it will wind up eating it over the course of a few hours
he'd start off feeling fine, but as the hypoglycemia progressed he'd start feeling dizzy, confused, then start having seizures and go into a coma
thing is, coma's gonna be about the halfway point and probably take an hour to kick in. the horrible thing about humans is that the body is good at keeping you alive well beyond where you can't survive, so it's entirely possible he would pass out after an hour of conversation (or less if he's stressed out like he was, depends on the rate the brain burns through sugar) and then stayed alive for another 2-4 hours, but comatose
the distance from unconsciousness to death is surprisingly large, so if we were to pointlessly apply medical realism to this fictional anime world with elves and magic, he would start off giving good advice, become irritable and confused after a while, start having seizures that would fling him from the chair, go comatose, and ONLY THEN be halfway to death, so when rudeus cremated him he'd still be alive
