Apparently there is a forum for engineering geeks and they went waaaaay farther into this topic than I will ever need to go. See link below
I assume most magazine manufacturers order their springs just a little long to "allow for set". For example the M&P Shield magazines that are initially so hard to load. Once the owner loads the magazine the first time the springs take that "initial set" and after that even compressed solid they don't approach the compression limit.
The following was originally posted on THR several years ago by a poster named "Walt Sherrill" . I have no idea if that was his real name or not but he was considered to be an SME on the topic of metallurgy and spring design.
Walt's Response
OVERFLEXING is the key. Most 1911 magazinesprings will have a long life, and the 7-round mag springs (like those in the WWII mags that were kept loaded) will have the longest life.
According to the experts, including Metallurgists (the engineers who work with and design metals) and others who use springs in aviation, auto, and space applications, flexing/working a spring will have little effect on it's working life unless, as it's compressed, the spring nears or exceeds the springs "elastic limit." (The "elastic limit" is the amount of compression a coil spring experience until it's damaged. Most guns springs (like tappet springs in cars) are designed and used in ways that don't approach that limit. The exception? A lot of very high-cap mags, recoil springs in sub-compact guns, some sub-compact mag springs, etc. In those more-specialized cases, the gun designers consider the springs "renewable resources" and spring life is sacrificed for additional rounds or function performed in a smaller space. When a spring's elastic limit is exceeded, the spring's metal begins to suffer from micro-fractures, and as the spring is used, the amount fracturing continues -- until the springsoftens and doesn't function properly. None of this has much to do with the HEAT generated by flexing, but simply has to do with how the steel's structure responds to the amount of flexing/stretching it experiences. Steel is a very resilient material. Aluminum, on the other hand, isn't -- which is why we don't see aluminum used in springs.)
The Rohrbaugh R9 had a recommended recoil spring life of about 250 rounds/cycles. (It had originally been about 100 rounds higher, but they lowered the round count after the gun had been out for a while. The springs weren't that expensive.). The springs might still work after 250 round, but folks didn't want to RISK a failure or poor functionality (like the inability to chamber the next round when used as a carry weapon.)
The R9 recoil spring was a very small springstuffed into a very small frame and slide, yet it had to still cycle a 9mm round, stripping a 9mm round from a mag and chambering it. That springdidn't last as long as a full-size spring in a full-size gun firing the same number of rounds.)
With most full-size guns, leaving the mag loaded might not make a difference. For some very high-cap mags, leaving them loaded might shorten mag spring life. If cycling springs alone wore them out, many cars with tappets wouldn't be running -- as those springs cycle many millions of times over a car engine's life. Note: Wolff Springs (in the FAQ area of their website) suggests, for hi-cap mags, that the owner download the spring a round or two during storage -- they say they'll last longer if you do. As noted above, however, not all mag designs require that.
Personally, I don't store magazines fully loaded except for in my carry gun and I rotate those every year.
www.thetruthaboutguns.com
. 
