There's not really a prescribed amount of mileage, conditions vary too much. People who live in cities with severely buckled streets (minnapolis, if I'm remembering correctly...) will tend to wear out shocks faster than people in cities with less buckled streets. People in extreme cold environments or who load their vehicles very heavily and drive in the desert will tend to wear out shocks faster than people for whom those things aren't true.
I'd be pretty disappointed if I didn't get 100k miles out of a set of shocks really.
If you're going to DIY the work, now or in the future don't forget that you will need either a 21mm crowsfoot wrench or the Thuren rear shock tool to remove the upper rear shock mount nut. No way to get at it with a regular socket or wrench, with the bed on the truck.
Probably the most popular shock replacement on these trucks is Bilstein - they make a number of different grades of shock for the 4th gen 1500 from the "HD" stock replacement shocks (yellow and blue) and 5100 (silver, front ride height is adjustable from stock up to 2.8" lift) to some remote reservoir rear shocks and the larger 6112 grade premium front shocks. The "HD" and 5100 shocks are constructed exactly the same as each other the only difference is the paint color on the HD's and the adjustable ride height on the 5100's
There's no shortage of other choices too, if you're not picky about it and just want the next set to last a long time with minimum cost you can't go wrong with KYB.
