wondering this for a while now?
There is really no limit to service life. Keep it clean and lubed and it will possibly outlast you. The system is comprised of many parts. Bolt, firing pin, gas rings, gas tube etc... They will wear depending on frequency of use. It all depends on the composition of the parts... You may have to replace some of the parts..
I've never life or limit tested one so... I don't know how many licks it takes to get to the center of that Tootsie Pop...