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

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).

After reading the answers I will never worry about speaker longevity.

Good.  Just make sure you keep them away from running water 😃.  Actually, some manufacturers cite maintaining medium ranges of humidity as a way to ensure long speaker life.  Sounds reasonable to me.

50 year old ESL57’s

45 year old Cornwalls’s

50 year old Dynaco A25’s

All still working just fine :)

The ears usually wear out before the speakers do. Given some sonic degradation over time, it is likely so gradual to even be noticed, if at all, or subject to the “used to it” effect.  Playing or swapping out will produce sonic differences. Is the result an actual improvement? Only your hairdresser knows for sure. Exhibit A, most of this forum is dedicated to sonic perceptions 🙃