I don't think you can generalize....a lot depends on the speaker manufacturer and their design philosophy. It can probably vary from as little as 5 years to much longer.
At one extreme are pro studios, where robustness is very important, usage is high, down time is a nuisance and consistency in response is expected over many years. For example, ATC provide robust speakers for the pro market and claim an upgrade path going back over 25 years. All of which is helpful in the case that something breaks or wears out;
http://www.atc.gb.net/news/2005/FrankWolf.html
This makes me tempted to generalize that it will be safer buying older gear built for professional studio use than older gear built for consumer use. However, even companies supplying studios can go out of business and shut down manufacturing!
Perhaps the safest advice in second hand older speakers is to stick with a popular model with many years of production, a good track record, and built from drivers/tweeters that are still widely used. At least there will be parts available!
At one extreme are pro studios, where robustness is very important, usage is high, down time is a nuisance and consistency in response is expected over many years. For example, ATC provide robust speakers for the pro market and claim an upgrade path going back over 25 years. All of which is helpful in the case that something breaks or wears out;
http://www.atc.gb.net/news/2005/FrankWolf.html
This makes me tempted to generalize that it will be safer buying older gear built for professional studio use than older gear built for consumer use. However, even companies supplying studios can go out of business and shut down manufacturing!
Perhaps the safest advice in second hand older speakers is to stick with a popular model with many years of production, a good track record, and built from drivers/tweeters that are still widely used. At least there will be parts available!