I've had the shocks for probably 50K mi and it still rides like new. They're all rusted out too, the Grand C. has 114K mi on it.
Tanks ad59. I drive very smooth, and have been told that many times, maybe thats it .
Do the bounce test. You push down hard on the corner, or jump off the bumper and see if the vehicle bounces more than once. If it does, you need shocks.
It largely depends on your driving habits and the road conditions,
also the quality of the shocks...you may never need to change them during your ownership time..