| A: How long is a ball of string?
Depends on what strings you use, how hard you press, what general technique you use, how much you play on specific frets, what material the original frets are made of, and how many hours you play.
When they wear down, have them dressed. When they wear out, replace - and there are different fret materials that last longer. You can also try different strings. Flats wear frets less quickly than rounds.
But bottom line: frets are perishable. They are like brake shoes on a car - they get used up, and then you replace them.
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