How long is reasonable before I should expect to see fret wire wear on my acoustic guitar?
i think that would probably depend on how much you play really.
If you just play it casually you could get 10 years + out of them before needing to do a refret. If hardcore playing, that time will be a lot less. If in doubt as to weather your guitar needs a refret or not, bring it in for an inspection.
10 years is a solid estimate for most general guitar players.