It depends on the materials used....some can go on for lifetimes. Foam surrounds rot and will need to be replaced usually within 20-25 years. Some caps will also need to be replaced at 20-30 years, others will go much longer. Ferro fluid will dry out in time and need to be replaced in 20-30 years. With some maintenance, many can live on as good as new.
My personal speakers are pushing 33 years old with components that are over 35 years old. The butyl surrounds and kevlar cones are in great shape, as are all the crossover parts. Solder joints all look good. If anything, the well flexed spiders and surrounds work better than ever. Even the paint and cloth grills still look great. The only thing I’ve replaced was ferro fluid in the tweeters.