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

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.  

@jimijam good for you, however, I did read this blurb on a site selling replacement polycaps for the AR-9 (I have a set and plan on opening them up to see which capacitors were used. Mine are circa 1982, so I don’t know if they might be Callins or Sprague, but I’ll find out).

Blurb: "Do you need these capacitors? Twenty years ago, when we began servicing these speakers, there didn’t appear to be any need. However, perhaps ten years ago, we began to find capacitors manufactured by Callins Inc. installed in other AR speakers that were way out of spec. That created concern for the AR 9 and AR 90 speakers and we began finding them in these also. In recent years, customers have been requesting this kit for their speakers, particularly needing them for the tweeter section.

However, even though some larger Callins capacitors may be found in the woofer crossover section, the impact does not appear to be as great. Now, if you have a late production pair of these speakers and find your capacitors to be manufactured by Sprague Electric, there is no need to replace them. We have never found a Sprague Electric capacitor to be out of spec."

Apparently the worry is that an out of spec capacitor could cause you to blow your tweeters by feeding more power to them than you think you are, tweeters that can’t be easily replaced, if at all.

I did have the surrounds on my drivers replaced 15 years ago with butyl ones that should last "forever", but was dumb and didn’t document the crossover components back then.

My early 90s NHT 2.5s and 1.5s have been great over all these years but I somehow had a surround 'rot' on a 1.3 speaker, not sure why; the other 1.3 was/is fine still.  My 1996 ML SL3 electrostatics with 10" woofers still sound great in my home theater.

I've got early B&W DM301s? 30+ years sounding fine as well. My brother's mid 90s Thiel 2.3s sounded superb two years ago when I last heard them.

I do think well made speakers, taken care of, will last quite a long time. More recent ones have the benefit of better engineered materials (if appropriate for the design).