Shocks and struts do not require replacing at specific mileage intervals like filters or spark plugs, but they do wear out and eventually have to be replaced. How long a set of original equipment shocks will last is anybody's guess. Some original equipment shocks may be getting weak after only 30,000 or 40,000 miles. Struts usually last upwards of 50,000 or 60,000 miles.
But when exactly a shock or strut needs to be replaced is hard to say. Because the damping characteristics of shocks and struts deteriorate gradually over time, the decline in ride control often passes unnoticed. So by the time to think you need new shocks or struts, it's usually way past the point when they should have been replaced.
One way to evaluate your need for new shocks or struts is to consider how your vehicle has been handling and riding lately. Does it bounce excessively when driving on rough roads or after hitting a bump? Does the nose dip when braking? Does the body roll or sway excessively when cornering or driving in crosswinds? Does the suspension bottom out when backing out of the driveway or when hauling extra passengers or weight?
A "bounce test" is still a valid means of checking the dampening ability of shocks and struts. If the suspension continues to gyrate more than one or two times after rocking and releasing the bumper or body, your shocks or struts are showing their age and need to be replaced.
Why Replace Them?
Weak shocks and struts won't necessarily create a driving hazards if you continue to drive on them, but there are studies that show worn shocks increase the distance it takes to stop a vehicle on a rough surface. Increased body sway due to weak shocks or struts can also increase the risk of skidding on wet or slick surfaces.
Worn shocks and struts also increase suspension wear (though marginally) but can have an effect on tire wear. If the shocks .
The reason why most people decide to have worn shocks or struts replaced, however, is to improve overall ride quality. If you're sick of bouncing and rocking on rough roads, a new set of shocks or struts will firm up your suspension and restore proper ride control.
If you're interested in performance handling, you can upgrade to premium "gas" charged shocks or struts. These are charged with high pressure nitrogen gas to help minimize foaming in the hydraulic fluid inside the shock. This lessens "fade" on rough roads and helps the vehicle maintain better ride control when cornering.
http://autos.yahoo.com/maintain/repa...ques102_2.html
for me personally, when the ride isn't comfortable any more or by at least 60,000-65,000 miles, i will replace them.
500 miles if U have the money. 
Linear Mode
