One rule of thumb is to have a spare recoil spring to compare with. When your spring that is in use has compressed two coils shorter than an unused spring, it is time for a change.
How long a spring lasts depends on a couple factors. What load you are using, and what gun you are using. The hotter the load, the more abuse the spring takes. The lighter the slide, the more abuse the spring takes. If you are using a compensated gun, the spring takes less abuse than an uncompensated gun. So, if you decide that you are going to replace the spring after X number of rounds, remember that this rule of thumb only applies if you are shooting the same ammo all the time. And it only applies to similar guns.
Another method to use only applies if you use a shok-buff. The shok-buff should last about 1000 rounds. If you start chewing them up more frequently, then the spring needs replaced.
When I buy a new autoloader, I change all the springs in the gun before I ever fire it. Wolff offers spring pacs that contain all the springs for a particular gun.
With the relatively low cost of springs, it doesn't make sense to ignore them. If you are not shooting typical loads out of your gun; real light loads or real heavy loads for that caliber, you might want to also try a Wolff calibration pac. This contains a range of springs that will allow you to try different springs to find the best spring for your load. I try to use the heaviest spring that will allow my gun to function reliably with that load. This gives you maximum protection for your frame. Once you find the appropriate spring from the spring calibration pac, you take that spring out and replace it with a new spring of that weight and return the calibration spring to the set, so that the next time you use the calibration pac, you have all the springs in the set.