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

The only problems I've ever had with speakers were tweeters blowing out. It actually happened to me a couple/three times. With my Celestion SL600's and my KLH 17's.  I never played my stereo very loudly so I have no trouble blaming the componentry. And then there were several times when ultra-weighty "audiophile" speaker cable pulled my petite mini-monitors off their stands onto the floor. More than once it resulted in speaker cabinet dents & dings. For a while I'd rope speakers to their stands with twine. Now I use Quake Hold Museum Putty.

@immatthewj  I'm sure you are right, I was answering the more generic question the OP posed in the title rather than the parenthetical one.

In general, a long time… well over 20 years… but, it depends. For instance I had a pair of ESS AMD 1Ds… which had relatively inexpensive woofers with material around the woofers exterior that wore out after about 12 years… similarly my B&W subwoofers needed the cone suspension material replaced after 25 years. The smaller B&W 805 Matrix speakers (in my home theater) sound and are great after 25 years.

I think if you are talking about high end speakers (not ESS AMD 1D… only good, not audiphile) thirty years or more is not unreasonable. But, as in most things, it depends.

@ghdprentice  , as a matter of fact, my speakers are the older 805 Matrixes.  I think I've owned them about 25 years.

(I did have to redo the cone suspension on my M&K sub twice in those 30 years.)

My 35 year old Spendors and 56 year old Quad ESLs still sound excellent.