Fan longevity is reduced under sustained high-duty-cycle - meaning that if you keep the fan spinning up at high (or at max) for long periods then yes it will shorten its life. Burn out the bearing, mostly likely, or perhaps the motor, possibly even seize it up through some sort of electrical failure.
I don't know the part/model numbers or specs for the fans ASUS installed in that machine. They're probably good or excellent (but not truly superior) quality, probably rated to last about 2-3 years of "heavy/continous" use or maybe 3-5 years of "average/normal" use. Lacking any specific details, the warranty on the laptop itself is probably a fair indicator of when the most failure-prone parts (like drives and fans, stuff with moving parts) are expected to start dying.
I wouldn't worry about 100% fan too much. It's probably got a good motor and liquid- or ball-bearing system, built to last several years. It costs maybe $10 or $20 to replace, it might even be the sort of part that's replaceable or upgradeable with aftermarket kits. It might die an early death (no sooner than 24 months from now) in the noble cause of increasing overall laptop performance and adding longevity to the $500 CPU that's permanently soldered onboard.
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