Right from Wolf Spring web sight.......
http://www.gunsprings.com/1ndex.html
5. Should I unload my magazines, rotate magazines, load with fewer than the maximum rounds? How often should I change magazine springs?
Magazine springs in semi-auto pistols are one of the most critical springs and the subject of much debate and concern. Magazines which are kept fully loaded for long periods of time, such as law enforcement applications, will generally be subject to more fatigue than the weekend shooter's magazine springs which are loaded up only when shooting. Magazine design and capacity also affect the longevity of the spring. Older designs where maximum capacity was not the goal such as the 7 round 1911 Colt magazines will last for years fully loaded. There was a lot of room for a lot of spring which reduced the overall stress on the spring. In recent hi-capacity magazines, the magazines were designed to hold more rounds with less spring material. This puts more stress on the spring and will cause fatigue at a faster rate. Unloading these magazines a round or two will help the life of the spring. Rotating fully loaded magazines will also help the problem somewhat but is not always practical. In applications where the magazine must be kept loaded, a high quality magazine spring such as Wolff extra power magazine springs, will provide maximum life. Regular shooting will verify reliability and regular replacement of magazine springs will provide the best defense against failure from weak magazine springs.