I've never heard of a "typical lifetime" for CV joints. It's probably something they should consider looking into. Just to have an idea. (say every 120k miles for example)
I would say it depends on your driving. How often do you use 4wd? How is it used. My burb has over 200k miles, and shows no symptoms in the cv joints. But then 4wd is rarely used.
Even still at the question of how often is 4wd used, the front wheels are not the only ones doing all the work. So, there is not a massive amount of strain ever put on the cv joints like there would be in a fwd vehicle.
I would think if you don't hear the tell-tale clicking/popping when you make a turn then the CV joint is probably good enough to repack.
Anyone else have an opinion on this, or some data they can share?