Glad to hear you have him propped up. We've kept Silkies we are treating inside in a cat bed if necessary to keep them from rolling around or falling and hurting themselves.
How old is he? What was his daily routine like before he was found like this? I'm asking what are the other birds like; do they treat him well, or does he get bullied; does he mix well with his flock mates? What does his diet consist of; does he eat a healthy amount of food, etc?
I wanted to provide a little more info than I gave before, and also say that I recommend veterinary care if possible. With that said, to answer your questions above,
Is it life-threatening? ANS: Wry Neck is not contagious, but it can be life-threatening to the extent the bird may not be able to eat/drink - seems you have that covered though. Diseases such as Mareks, which might present symptoms like this,
are contagious, but you're likely going to have flock issues if that's the case. Wry Neck seems more likely if he's the only only one acting odd. Silkies are more prone to Wry Neck too, so I'd start there.
Don't discount the possibility of head trauma from a fall, running into something, getting pecked by another bird, etc. Make sure to examine his head carefully to see if you can see any signs of trauma (blood, broken skin, etc).
How long might this take? ANS: Like Wyorp Rock said, it can vary. Most of the cases we've dealt with show improvement in as little as 48-72 hours, with some showing improvement the same day. In our most extreme case, it took 2 months. I even know of another user here on BYC that I coached through a Wry Neck case that almost gave up after something like 3 weeks of treatment, when suddenly the Silkie being treated finally responded. The short answer: IT VARIES... :-\
When we're treating Wry Neck, we give lots of healthy treats too (grains, mealworms, tomatoes) since the bird is usually not eating nearly as much as it would normally eat. The more you can get the bird to eat, the better. We usually make a mash of moist Feed mixed with Yogurt, Vit-E, and Fried Egg mixed to moist, easy to eat and tasty servings. An alternative to Vit-E/Fried Egg to achieve Vit-E uptake, Selenium and Vitamin E for Goats comes in a gel from TSC stores and is super-easy to mix into moist feed if you think that might be easier for you - it works well for us...
Good luck with your boy! Keep us updated