"Being repeatedly compressed and relieved is what wear springs out. Not being compressed and left." With respect sir, this is a common belief but has not been my experience when it comes to large capacity shotgun magazines. There is a lot more compression taking place than in any handgun magazine. Ask some police armorers about loaded shotgun springs weakening from long term compression and storage. These guns sit fully loaded and only get fired maybe once a year (if that). When tested most of them will feed the first 3 or 4 rounds and then start choking. New magazine spring restores 100% feed. Those springs weakened just from full compression over enough time. Undoubtedly the quality of the spring has an effect on this.
My duty shotgun, an 870 with a six round magazine, has been fully loaded for 12 years except for firing 4 X a year and cleaning. When should I expect the spring to fail or "take a set"?