Over time you will gradually hear the sound quality diminish. I have only had to change out my stylii once and it was very noticeable when it was necessary. I had a pair of used needles that I got with my turntables that were so bad they didnt even properly reproduce sound off the records. I can't tell you an average lifespan since I don't know what needles you are using or how much you use them, but I can tell you -- when they need to be replaced, you will know to replace them

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Helping girls make bad decisions since 1979

When the sound quality diminishes it's too late already, by the time your styli reach that state they're well underway of erasing a memory that used to be music.

You can also tell when too much dust is starting to stick under your styli. If you pick up your tonearm and the dust keeps sticking it means it's angle isn't straight anymore. If you got a needle microscope (like I do) you can tell instantly.

Most needles tend to last about 6 months of heavy use. Obviously, the more expensive/better needles will last a bit longer, and the cheaper ones a bit less, but you get the idea.