All I saw was 246k miles, and I can easily say it wont last you more than 2-3 years, depending on how much driving you do. Now, I am not a chevy owner, nor have I ever driven a chevy, but from common knowledge knowing that a Silverado is a big truck and since its older, the gas wont be as good, but the extra weight won't help the longevity of the car.
Also, include how much driving you do, because if you do a lot, then you got maybe a year or two, but if you only really drive to work and you live close to your drive, I can see a god 3-5 years as long as you take care of it.
But I think that they are 15c/20h. I got that from a new 94 model, I don't imagine there is much of a difference though.