Interesting question. I've never heard of changing a whole set of piano strings, although I would think they must deteriorate over long periods of time. I've had my piano now for about 17 or 18 years and have never done more than have it tuned and the mechanism lubricated with graphite powder. Maybe heavily used concert grands require string changes periodically if they're used for classical performances, but I've noticed no degradation of sound in mine.

david