How long does it take a decent quality speaker to "wear out"?

@dekay  , I remember you mentioning that back when I was inquiring about 5AR4s.  I am hoping that I am not as hard on rectifier tubes as you are on glasses.  

I own a set of DQ 10's.  Bought them from an electrical engineer.  They are still 100% original as far as I can tell with the exception of a re-foam  He told me they were never driven hard.  His wife wasn't too fond of them.  They sound fabulous.  Crossover looks original with the yellow DQ caps....

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"

Ask a guitar player who owns an original "tweed" Fender amp from 1954 or so. 

I have the same question regarding my now almost 25-year-old Platinum Audio Solo speakers.  For several years I've been thinking of replacing them with something new, but only because of their age.  They still sound terrific.

The speakers were designed to be played with the grills on and I admit I never bothered to remove them until last year (you have to pry them off).  The drivers look as if they were brand new.

I wish I knew the quality of the crossover parts, but I suspect the crossover was well built considering most of the rear panel of the speaker is a heat sink.