this is taken directly from tootechracing.com lot of reading but it explains the answer to your question sorry i cant post the chart but you could go to there site and check it out.
Do Suspension Springs Wear Out or Why Do Springs Break??
In our Motorcycle world springs rarely break. A well designed steel spring will basically last forever. How long a spring lasts is based on how much stress is produced in the wire when it is compressed to it's minimum length AND how many cycles (times) it will experience this load. Engineers have charts to describe how many millions of cycles a given spring material can take at a different levels of stress before they fail. (See chart below) As the stress in the wire increases, the number of cycles until failure decreases. The larger the spring wire diameter is, the less stress the wire will experience for a given spring rate. Motorcycle suspension springs use relatively large diameter wire for the loads they see in service thus they seldom experience enough stress in the wire to cause breakage. Some aftermarket companies try to use cheap wire which can't handle high stress levels without severely shortening their life expectancy. But most of these companies have seen the error in their ways and now I am not aware of any companies with chronic spring breakage problems.
The chart below is used to predict how many cycles the spring can experience before it fails. First we must determine which chart line to use. This is determined by analyzing how the spring will be used. The second picture down on the left side shows a spring cycling from no load to it's maximum load (or maximum negative stress). Our springs have almost no load at full extension so the second picture best describes the stress pattern our springs will see (from 0 to max and back to 0). This stress pattern is considered normal, so we use the standard graph marked "R=0".
The chart on the right is for some mild steel. If it were a graph of spring wire it would look similar but the allowable stress loads would be higher. The X axis (horizontal axis) on the bottom denotes the number of cycles the spring will experience before it breaks while the y axis (vertical axis) describes how much stress the wire receives during each cycle. If you follow any one of the lines you will notice that as the number of cycles increases, the allowable stress the spring wire can handle goes down. Follow the "0" line. Note that if we want to design for only 1000 cycles, our material could handle about 105,000 PSI stress in the material before it failed. If we want to design for 100,000 cycles, we can only stress the wire to 80,000 PSI. Notice that at about 500,000 cycles the line levels out at a stress level of about 75,000 PSI. This means that if we subject this material to a maximum stress of 75,000 PSI, it can be cycled indefinitely and never fail.
If a steel spring is designed properly (utilizes the proper wire size) for a given application, the spring will never experience stress levels higher than the horizontal part of the line. Thus a properly designed spring will last forever. This is a unique property of steel only. For instance aluminum, titanium, and other exotic materials continue to "age" regardless of their stress levels (there is no horizontal part of the line). Because of this constant aging, engineers must use large cross sections to keep the stress levels in the material very low. As long as the stress levels are very low, these materials can last a very long time. (Think about this as your aluminum bars get to be a few years old!!!)