A SIG with a stainless steel frame will last longer than a SIG with an alloy frame. Of course the downside of the stainless steel frame is the extra weight which is substantial but for a HD/range gun it may not matter to most and the felt recoil is less.
However IF you take good care of a SIG with an alloy frame meaning to keep the frame rails well lubed ideally with grease, changing the recoil spring every 3000 - 5000 rounds, and changing the breech block pins every 3000 - 5000 rounds on folded carbon steel slide SIGs you should be able to get at least 60,000 rounds out of it and some get a lot more with reports of over 100,000 rounds. Of course by 60,000 rounds you will have probably payed about $18,000 in ammo costs.
Here is the SIG maintenance schedule courtesy of sigfreund - member of sigforum who is also a SIG armorer.
believe this may still be of interest (original information from March 2008).
The following is SIG SAUER's current parts replacement schedule (numbers of rounds), with some comments of my own:
Barrel: If the pistol exhibits keyholing or unacceptable accuracy.
Decocking lever spring: 10,000
Extractor: 20,000
Extractor spring: 20,000
Firing pin: 20,000
Firing pin spring: 20,000
Magazine spring: When magazine fails to lock the slide open. *
Recoil spring: 5,000 †
Roll pins for breechblock retention (pins, HD): 5000
Safety lock spring: 20,000
Slide catch lever spring: 10,000
Takedown lever: 20,000
Trigger bar spring: 10,000
Mainspring (hammer spring): SIG SAUER evidently makes no formal recommendation about the mainspring, but lbj was told by customer service that guns with 40,000 rounds are still igniting their ammunition with the original spring. If ignition problems do develop and can't be determined to be due to other reasons, the obvious course would be to try a new mainspring.
* I also recommend replacing the magazine spring when ammunition is not held securely. This can occur before slide lock failures.
† I had seen an early recommendation here that the recoil spring be replaced after 3000 rounds, and that's what I follow.
For the armorers: SIG considers the extractor used in P220 stainless steel slides to be a "sacrificial" part, and that it should be replaced after it's removed from the slide
