How long do speakers last?

Actually if your speakers have foam surrounds they could have rotted by now. You should inspect them because if you run them without the surround much you can damage the voice coils. New surrounds are cheap and easy to replace. Once done the speaker is as good as new. I'm presently listening to a pair of Dynaco A 25's that were made in the early seventies. Rubber surrounds so no rot. They work fine and sound pretty good too.

These are dcm time windows. (not sure model)

As was stated above it depends mainly on the surrounds. If foam then less than 10 years same for butyl rubber and the like. I inherited some 1959 JBLs which work fine, however they use fabric surrounds and compression horns. You need to take that sock off unless some one knows what type of surrounds you have. Most used foam or rubber by then. Therefore I would recommend getting them remounted.

the foam surround can just get worn out, resulting in a less 'quick' sound.

also, how much do you listen to them? 1 hour vs. 8 hours/day might be a factor in longevity...

The older a speaker is that much better it will sound, but Capacitors and surrounds will deteriorate in time, if they are good rubber surrounds on the woof's this will not be a problem really, but would still suggest replacing caps for another level of performance, not really because they are necessarily failing, although they do, but newer caps are far more advanced and better material, (with musical results not just scientific results) now available than anytime 10-20 years ago. Beyond that if you do not toast a voice coil or damage them they could last more than a lifetime as long as the glue on the cabinet don't let go, or the wiring does not corrode to some crazy point.