Shock absorbers usually wear out gradually, but there are many factors that can affect the rate and severity of wear. A general rule of thumb is to check your shocks if your vehicle has about 25,000 miles on it. After that, they should be checked every 6,000 miles. Check for signs of seepage on shock absorbers (oil dripping out of the shock) and test them by bouncing the vehicle up and down. The vehicle should stop bouncing when you step away.
If you let your shocks become worn, you could be bouncing, swaying and rocking all over the road. Worn shocks can lead to excessive bouncing when you drive over dips or rough roads, which can accelerate the wear on your tires, not to mention your stomach. Worn shocks can also cause excessive rocking when you stop and can cause your vehicle to sway whenever you make lane changes.