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

In 1972 when my wife and I were both in college we bought a pair of Bose 901 Series 1, the ones with the drivers having corrugated surrounds.  Now half a century later the speakers are still going strong in our son's basement party room.  The chintzy little equalizer long since bit the dirt and was replaced by an after market equalizer. 

Loudspeaker materials usually don't last forever but it's hard to wear out most speakers.

@mijostyn: You are saying that "modern ESLs can be virtually immortal" but as far as I know at least the Martin Logan panels are said to have a limited life expectancy?! I never understood why. Electrostatic headphones can last many decades...

Speaker enclosures can wear out as well. Veneers can loosen, hardwoods can warp. Proper, rugged woodworking should see them last decades, however. Foam surrounds deteriorate over time, though newer types (from the last two decades or so) last somewhat longer, depending on their specific implementation. Not only direct sunlight and certain atmospherics can be their enemy over time, but also prodigious excursion. The cone and surround material that has stood the test for the longest time are paper cone variants and cloth surrounds. Paper cones, even older ones, usually have been given a moisture-repellent treatment, certainly pro units that can see use in harsh, outdoor conditions, and cloth surrounds - be they treated or not - last close to forever in fairly stable environments. Indeed, pro drivers are typically built like tanks. Put them at use in domestic environments and they're cruising along without being thermally or mechanically challenged, while sitting in relatively stable indoor humidity and temperature conditions. Rubber surrounds, even modern varieties, can become inflexible and crack, while other (butyl) variants last decades without issues. Rubber surrounds can be treated to remain flexible. Compression drivers run like forever. Some ESL's are very durable, though newer Quad's have a history of depressing failure rate, also being quite sensitive to the environment where they're used. Overall, it would seem the best way to ensure longevity of the mechanical parts in speakers is frequent use over the years, while generally avoiding driving them to their limits. 

The life expectancy of electrostatic panels depend on the specific models and operating conditions.  The Martin Logan panels do not arc and they do not develop holes in the diaphragm from arcing.  In that respect the panels are MUCH more durable than are the panels on Quad speakers.  However, the Martin Logan panels can be susceptible to to the wires that bring the charge to the panels corroding at the point where the wires attach to the panel.  I know someone who suffered from this problem because he stored his speakers in a garage that gets quite humid in the summer.  Owners in humid Southeast Asian countries also have reported similar problems with Martin Logan panels.  Otherwise, the Martin Logan panels seem to be quite long-lived.