i have two Hollywood HSL12SE subs i bought in 1996. they are in perfect condition dispite a lot of use. sold for $75 each in 1996.
i recently purchased and installed Oz Audio speakers from 1999-2001. they're great (sold for $800+ new).
many things affect a speaker's life, it's initial quality, how it is used, stored, and it's materials are the biggest ones. if the materials rot over time, then the clock is ticking. but most modern speakers are made of 20+ year materials. so it mostly depends on storage (no damage, etc.) and use (no abuse). a quality speaker will last.
i have home speakers from the 80's that look and play like new. the biggest issue before that was foam surrounds (everything else lasted).
as to your question - you should get at least five years out of speakers, 10 or more is easy to attain. regular use at low-to moderate volumes is best for longevity, but a good speaker can take a lickin' and keep on tickin' (so to speak).
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