The easy way to do it would be to purchase an application on your ipod from Service Spring co. you can plug in all the measurements,
1. Wire diameter in inches
2. Inside coil diameter (usually listed on plug, but most common on residential doors is 1&3/4" aka 175 on the plug, and 2" or 200 on plug.
3. Length of spring, just the wire, not including the plugs.
Their is as of yet no site online that can tell you how to do it. and the Service spring co. app is not as good as it could be when trying to calculate life cycles. For that it is best to have a chart with ratings for springs with different total turns put on the spring. For example a 7' tall door usually requires 7.6 turns, while an 8' tall door requires 8.5.
This all goes out the window if you have a easy torque spring system from Wayne Dalton or other crappy manufacturer. Enough of my bias though, it is rooted in fact.
If you have a 32" long spring, with a 2' ID (Inside Diameter), and .250" Wire diameter, this spring on a 7' tall door would last approx. 15,000 cylces.
If you moved up a step in wire size, you would need a 40" x 2"ID x .262" spring, this spring would last approx. 30,000 cycles.
I am going by memory here, so if you would like further calculations let me know.
Things to consider before upgrading spring life cycles. (because depending on whom you buy from, it may cost a lot more money)
1. How good of door do you have? is it going to last a long time.
2. The springs you have should be at least 10,000 cycles, but may be less, this last most people 3 years if they open it 10 times a day. How long do they need to work.
3. Would you consider using the front door more often, the front doors handle is usually rated at 200,000 cycles and up.
let me know if i missed anything.