It's a pretty subjective thing. Some guys will get 40,000 while others 110,00miles. A lot depends on how you drive, the kind of roads you drive on and just plain old luck where some last longer than others.
You probably already know this but just in case...Bounce the truck at each of the corners. If the truck continues to bob up and down after you stop, then you need new shocks. If the truck settles quickly, then your shocks are fine.
I have 84,000 on mine and I am just starting to notice the truck bouncing a bit more than normal.