doubt there would be a limit? all the same parts are moving whether "most" people are in 4wheel drive or not.
if you are in a position where you do not need 4 wheel drive, but still use it, then most likely parts will be stressed more and wear out or break faster, or in relation to speed.
if you are in a position where you do need 4 wheel drive, then you should be more worried about killing people than wearing out parts.