The useful life of a set of grinder burrs is going to depend on several factors, even if we don't know exactly how many pounds or kg of coffee should be run through a particular set of burrs before they are changed.
The most obvious factor is the amount of cutting surface, which for planar grinders will equate to the burr diameter. It is well known that small planar grinders such as the Rocky need to have their burrs changed often, every 1-2 years, if I can believe what I read on these forums. As the diameter of the burrs and the size of the grinders increase, the need to change burrs is reduced. At 64mm diameter and beyond one is really dealing with a commercial grinder intended for a business, and these will have hundreds of pounds of beans run through them before even a conscientious establishment will change the burrs.
The original freezing study
/store-coff ... tails.html had one small "side experiment" having to do with comparing two identical 64mm diameter (burrs) Cimbali Junior grinders, one of which had brand new burrs and the other of which had 100 to 150 lbs of coffee already run through it. The hypothesis we were testing was that changing grinder burrs more frequently than usually recommended would have an impact on taste, and at least in this blind tasting experiment, this hypothesis was not proven in 64 paired shots tasted blind by 4 different tasters.
Large conical and hybrid grinders have large grind surfaces and I believe that in home use it is going to be well more than a decade before any consideration would need to be given to changing the burrs on such a grinder in home use.
Another factor that might deserve consideration is the type of beans that one runs through the grinder. Some of the green beans that are regularly available, such as some dry processed Africans, come with a "free bonus," chunks of rocks and concrete mixed in with the beans. If these beans are commercially roasted, virtually any high end roaster will have apparatus for "destoning" the roasted beans so that what you receive in your bag of roasted coffee will not include these impurities. Home roasters do not have destoners and other equipment for eliminating these undesirable things, and unless one is careful at examining the roasted beans, it is always possible to have some of them run through the grinder, which can't possibly do the burrs any good. So, a home roaster might want to change his burrs out more frequently.
ken