Let's go back to the barbarians at the gate analogy, shall we? The longevity of your walls of defense would depend a great deal on a number of factors. How often are the barbarians attacking, and how much damage do they inflict on the gates each time? How well do they hold up to the weather, and what are you doing to maintain them?
Pretty much the same holds true for the longevity of your vehicle's shock absorbers. Remember, whether you use gas-charged and conventional shocks, these devices deliver improved handling and a smoother ride by converting the kinetic energy of suspension movement during the drive into heat energy that dissipates through the hydraulic fluid in the shocks. Like the walls of an embattled city, there's only so much energy these shocks can absorb before they finally succumb.
As kinetic energy wears shocks down, it follows that more suspension movement has a negative effect on shock absorber life expectancy. If your morning commute consists of a smooth, unobstructed drive across level countryside, then your vehicle is probably enduring a minimum of suspension movement. Throw in some curves or a little stop-and-go traffic, and you have a lot more movement (and kinetic energy) slamming through those shocks. From there, gravel, hills and other road conditions only add to the stress on your vehicle's shocks, potentially subtracting from the typical 50,000-mile life expectancy. Even your style of driving and specific wheel and tire modifications can have an impact.
In addition to suspension movement, regional weather conditions and road contaminants can also take a toll on your vehicle's shock absorbers. After all, these are external mechanisms and regular drives through saltwater, sand or rough gravel roads can further wear your shocks down with abrasions or rust.
If you just drive the car to church on weekends, then this may all sound like great news -- not so much if you fill your Saturdays with muddy exertions into the wilderness. Luckily, you can take several steps to help maintain your defenses against all the road can throw at you.