Not only do shock absorbers play a big role in how comfortably your car rides, they're also an important safety item. Worn-out shocks are a hazard - to you and to other drivers who happen to be near your bouncing, swaying metal projectile.
Because of different drivers' different cars and driving habits, its impossible to predict how long shock absorbers will last. The original shocks on my small station wagon were worn out after 15,000 miles; the heavy-duty shocks on my van are still performing satisfactorily after 35,000 miles.
If your shocks are completely shot you can probably tell from the way the car handles. Bumps and ripples in the road make the car bounce and sway excessively. Driving a car with worn-out shocks is like driving a waterbed on wheels.
The best test is the bounce test. With the car parked on a level surface, put your hands on each corner and bounce the car up and down a few times. On a downward push remove your hands. The car should rebound once, then stop. If it continues bouncing, the shock on the corner should be replaced. And, as a rule, if one shock is worn out, its three brethren are too.
Or crawl under the car and eyeball each shock. If one's wet, hydraulic fluid is probably leaking out. Once the hydraulic fluid leaks out, a shock loses its ride-smoothing capabilities.
When buring new shocks should yosmoothing capabilities.
When buying new shocks should you go for standard shocks or heavy-duty shocks? Generally the rule of thumb here is: if you do a lot of trailer towing or carry a lot of passengers and luggage regularly, heavy-duty shocks are the answer. If most of your driving is just with a passenger or two, standard shocks are okay.
You can replace your own shocks and save the installation fee. But remember, when elevating the car, use a jack stand to hold it up. Don't use a bumper jack. If the bumper jack slips while you're under the car, you won't like what happens.
And keep the suspension compressed when unbolting the shock so that the coil or leaf spring won't suddenly expand, causing injury. By letting the car's weight rest on the rear axle (via the jack stand), or on the lower A-frame (with the front wheel), you'll keep the suspension compressed.
To remove a shock simply spray penetrating oil (you don't have to but it often helps) around each mounting bolt, let it soak a few minutes, then remove the mounting bolts and pull the shock out. If you run into a problem where a mounting nut or bolt is frozen, try to chisel if off. If that doesn't work, you'll have to ask a garage for help.
After unbolting the old shock, pull it out and put the new one back in its place. Place the new shock in in its upper mounting bracket, then compress it until you can attach the bottom mounting bolt(s). Tighten the bolts and you're done.