Life expectancy for speakers?


Can anyone tell me the rule of thumb for life expectancy on speakers? When should drivers be replaced? Under normal operating conditions with "average" use, should one expect 10, 20, 30 years? Lastly, what drivers tend to wear out first, tweeters, midrange, or woofers?

Thanks,
ethannnn
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If you don't over-drive or under power the speakers, everything except the woofer surrounds should last a long time. My local speaker manufacturer/rebuilder recently told me the much advertised Sharper Image washable (no filter) air cleaning machine generates ozone. He says this will appreciably degrade foam woofer surrounds.
I wonder how long before this becomes common knowledge?