Eric gave a good technical answer above (and i agree). For some practical data points, i'll venture that i had a pair of Boston A40s in the basement where the surrounds turned to dust with minimal use an 10-12 years - fast. One of my main pairs of speakers still in use, went into use in roughly 1994 and is played daily to this day - and appears every bit as good as the day i got them. They happen to have MB quart drivers with poly surrounds. i do expect that how hard they are played, temperature, humidity and light all come into play as well. but its kinda liek solid state electronics - aside from the electrolytic capacitors - their life is "nearly forever"
How long does it take a decent quality speaker to "wear out"?
After all, they do have moving parts and capacitors. . . .
(I suppose that "decent quality" s a relative term. FTR, I am running a pair of the older B&W 805s, and for all I know, they may not actually meet the criteria of "decent quality.")