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

Depends a lot on the usage and the environment. A lot of my gear that has always been in the house and under climate control has lasted 37+ years so far with only minor service, not due simply to age. A few caps in amps and some speaker drivers have been replaced, but very few issues. Some items that were kept in the garage did not fare as well. Foam coming apart, cabinets deteriorating, oxidation on the electronics internals, etc. I have friends that bought cheaper stuff and it was a total loss in a few years. Hard to say how long they last but buying good quality helps things last longer.

Matthew:

The old production 5ar4's were a good investment (get them while you can).

I purchased the remaining stock of the Benji Frank eyeglass frames I wear (90% off closeout) and they are now unobtainable @ any price.

Though they are durable frames I'm down to 9 out of 11 pairs (hard on glasses).

DeKay

@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"