In my first S&W 3rd gen armorer class the 2 instructors recommended replacing recoil springs based upon observed function and/or free length of the spring. This was with the springs used by the company, as they were familiar with those and had tested them over time. (The springs made by other spring companies might have a different length even when new, let alone once being used, due to the number of coils, length of each winding, wire gauge used, heat treat, etc, which might make any observation of "free length" meaningless compared to a factory spring.)
I think it was in the next recert class where I first heard the recommendation for replacing recoil & mag springs either every 5 years of service use, or every 5,000 rounds fired. Note that this was a verbal recommendation, not something written in the manual. It would ultimately be left to the individual armorers to determine what replacement interval they felt was appropriate for their own needs, in the conditions, circumstances and service needs experienced by them.
When asked where this recommendation had originated, we were told it was combination of feedback from various sources (engineers, LE user reports, etc), as well as some new requirement for providing more specific guidelines in training materials and classes. A legal liability/training influence was also mentioned. Of course, it was also mentioned that the use of higher pressure ammunition (like 9mm +P & +P+) could shorten the recommendation interval.
It was also mentioned that these recommendations were on the "conservative side", since they were intended to be offered to LE/Gov users of the guns, so safety and optimal functioning was a primary consideration. A "better safe than sorry" thing to keep in mind.
Over time it was usually discussed that the 9's seemed to experience less wear and tear and general stress than the harder recoiling .40 S&W, and the .45ACP sort of fell somewhere between.
The introduction of the sub-compact guns, like the Chiefs Special series, with its short flat wound recoil spring and faster and more violent cycling, didn't change the verbally offered recommendation for replacement intervals. However, some occasional discussions with other folks at the factory seemed to produce occasional suggestions that users might benefit from replacing the shorter springs a little more often. Nothing hard and fast or in writing, though.
Now, other gun companies have offered some different recommendations for such recoil spring, or recoil spring assembly (RSA) replacement, and even some of those have seemed to have changed or evolve over time. Some were/are a bit hard to try and pin them down when it comes to a specific usage/service interval, and some aren't. I kinda liked the way one company started telling armorers that springs are wearable parts and might require attention at some point. The armorer could decide when that might be.
Back about 10 years ago, one company offered (in writing) a recommendation for RSA replacement for LE .40's based upon a rounds-fired count ... and/or an inspection of function during hand-cycling/field checks ... and/or based upon observed functioning during
each range session ... or sooner, if needed. That ought to cover all possibilities, huh? Mag spring replacement also had a round-count recommendation interval ... or at least at
every other RSA change ... or sooner, if felt needed. All bases covered.
Some have gone back and forth over time, too. Some don't seem interested in even making recommendations, other than to check and replace the recoil spring (or recoil spring assembly) once the pistol starts to exhibit signs of a weakened spring. They leave it up to the armorer/user to determine when and if it becomes a problem. (Hopefully, that occurs on the range, and not when it's desperately needed.)
In my Colt Model O Pistol (1911) armorer class the recommended replacement interval for recoil springs in the 5" guns given, verbally, as somewhere between 1800-2500 rounds, and as short as 1200 rounds if +P loads were used. I discussed this with a well respected spring maker one time, and he said that for
duty/defensive 1911's even their high quality recoil springs probably ought to be replaced after 2500 rounds (we were discussing standard pressure loads, as I recall).
I've seen what can sometimes happen to some duty Colt 1911's when users let the recoil and mag spring replacement intervals go too long. One other 1911 armorer told me he started seeing some broken slide stops when they ran their 1911's, with +P training & duty loads, for a bit too long. Go figure.
Well, a little soft shoe might be fine, right?
One thing I remember was when a friend at another agency (their senior armorer and instructor) told me how he was unable to get the budget to replace their recoil and mag springs for their 4006's. He was told that since the factory didn't put their 5yr/5K rounds fired recommendation into
writing, that as long as the guns weren't exhibiting functioning problems, the money wasn't available for him.
Well, it was just about as his agency's shooters reached the 7 years of service point that their 4006's suddenly started to exhibit a noticeable number of feeding stoppages. Fortunately, it was noticed during some regualr training, and not out on the street. He finally got his money for new springs rather quickly.
I guess looking at it one way, that 5yr/5K recommendation was seemingly somewhat conservative, since the .40's at his agency were able to keep running for training and quals out to just about the 7 years of service point (I don't think the "average" officer fired a thousand rounds per year, but I can't remember the number he once told me.)
Now, our early 3rd gen 9's ran for a longer time period before we started seeing stoppages related to obviously weakened recoil or mag springs, but that's another story.
Personally, I like to replace the recoil springs in my own CS45 and CS9 much sooner than 5K rounds, but that's just a preventative maintenance thing, for me. I tend to order recoil springs 5-10 at a time. I let my 3913, 4513TSW & 4013TSW springs run somewhat longer than my CS guns when it comes to a rounds fired count, but I still like to err on the conservative side of things, to try and avoid the potential for unnecessary battering. I also monitor functioning at each range session ... and still peek at the free length of springs as they see increasingly longer use (old habit) and look for signs of wear that might be related to weakening springs during inspections, noticeable changes in ejection patterns, etc.
Bottom line? Recoil and mag springs are relatively inexpensive. A
lot less expense than frames, slides and barrels.
Just some thoughts, having listened to some different gun companies and a couple of spring makers over the years.