How long do speakers last?

Until I get tired of them.

They can last a lifetime, but you may need to replace a driver or components over the years. Some plastics, foams, glues, etc degrade over time regardless of the conditions they are kept in, so not uncommon for things to fail over decades. I still have my Polk SDAs bought new in 1987 and have only had to replace a few drivers. I've gone through a number of other speakers over the decades as well and rarely any issues, and they have always been kept in an air conditioned house as well. As long as you are not obsessed with getting a perfect OEM original replacement part when needed, you'll be happy. That said, replacing components sometimes provides an even better performing component compared to the OEM part from decades ago.

My brother has a pair of Von Schweikert speakers that are 30 some odd years old.

The VR 33s are an excellent design intelligently made to keep the costs down. The sock instead of a veneer is an example of this. The sock can easily be replaced by any thin elastic fabric available at any fabric store. They are reasonably efficient. Any good 100 watt/channel amp will drive them to ridiculous volumes without any clipping.  The surrounds are butyl rubber and will last forever. The crossover caps may need to be replaced eventually but that is no big deal. Learn how to use a solder iron.

If you like the sound and the price is right go for it. Getting better speakers new would cost a lot more money.

@secretguy , Only until you work your way up to the right speaker and there is a "right" one for everybody..... if you can afford them.

2 years ago I sold my Clearfeield Continentals, which were Von Schweikerts when he worked for Counterpoint. They still sounded fantastic.