If you don't abuse them they should last forever, subject to the caveat that some speakers need to have new surrounds installed.
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It's not so much a question of how long it can last, but how long will it perform optimally? Crossover component can go out of spec, driver surround materials can harden, connectors can corrode, magnets can weaken, etc. I would estimate 20 years as an effective life. Fortunately everything that can go bad can be readily and fairly cheaply fixed bringing performance back to day one new.
too many variables. in theory they should last forever. typically people managed to physically destroy them ( kids fingers spilt drinks, moving house etc.) before you wear them out. age probably hurts them more then use itself. stiffening adhesives & cracking will happen much faster if they are rarely used as opposed to frequent use keeping them limber for longer. cold climates and varying temperatures will hasten the degradation of the adhesives.
Hot Tip - ensure your speakers are never exposed to sunlight. a friend had one that copped morning sun for about 2 hours through a window and it needed new trifts in 4 years the other one looks brand new.