How long before hedgehog dies of starvation?

So, unfortunately, I have reached the point where my hedgehog is very near the end of his life. I've had him since he was 1 month old and was and he is turning 4 years and 8 months old in a few days, and has been having health issues for the past few months. Apetite has been a huge struggle recently, and I think I'm pretty much on the final straight, which is also what vets say. We've tried every reasonable option, and it just can't really be fixed. At his healthy life weight, he was + - 500g. Now, he just hit 400g and it's going down a few grams every couple days. He does not want to eat himself. I've been syringe feeding him the past days, but that can only help to a certain point.

So, to ask the question bluntly. If anybody has had experience with this, until what weight do you think he'll be able to drop until it's just too little and he passes?