Good speakers have been known to last a lifetime. That's why there are 50-year old ++ speakers out there. And a check with the surplus speakers at pier will confirm this as there are many 10-30 year-old sansui, pioneer and technics speakers still in surprisingly good conditions. Ofourse reasonable care may have contributed to their heirloom lifespan and will certainly do a lot to extend the life of your current speakers.
Turning off powered speakers, whether for the PC or something else, has more to do with conserving power consumption for the built-in power amps inside, not to protect the speakers. It's just good sense to do that when not using these appliances.
I know of no meantime between failures (MTBF) as far as speakers are concerned. Using the speakers even 24 hours a day need not shorten the life of your speakers. But if the usage results in the insulation of the speaker voice coils gradually melting due to excessive heat, then it will certainly break down one of these days. Speakers also generate heat. That happens when you feed the speakers large current (loud volumes) on a continuous basis. Or it could be due to exceeding tolerance limits that may be imposed using speakers designed with less durability in mind. You can feel the heat on the speaker magnet case, but since you normally would have no access to that, just feel the speaker cones if they are getting warm to the touch, that should tell you to give the speakers a rest.
In general, moderately powered amps should go together with moderately powered speakers. But speakers can be driven by as low as a 1 watt amp or receiver. When using amps with lower power rating than speakers, it's very tempting to max out the amp to its fullest volume, since, you are confident the speakers can handle that. There's actually more danger in that as a maxed out amp can generate clipped waveforms that will fry a speaker much faster than overdrving it with a more powerful amp (which should also be avoided.) Tweeters often are the first to go in situaions like these as they generally have less tolerance and will fry easily with distorted signals in the high range. Boosting the treble controls at loud volumes would be the easiest way to fry tweeter coils.
I've read on the net that paper-based speaker cones would benefit from being moistened with a cloth lightly dabbed in baby oil at least every year to preclude the material from drying and warping. Haven't tried this so I can't say much about it.
Just at the top of my head. And, just like Abad Santos said, see if you can keep small kids away from speakers. I've had dimpled speaker cone centers from curious little fingers.
