How long do speakers last?

Another vote for Bill Legall at Millersound. A wonderful man with a lifetime's experience repairing speakers. Not just surrounds, either, but spiders, voice coils, everything. Very reasonable prices, too.

I have a pair of Polk Audio Monitor 10's that I bought brand new in 1987 that are in mint condition and sound like they did the day I first hooked them up.  Never been opened up or worked on and all original components.  They have always been in a non-smoking house, and in a room that is always used (so well ventilated and climate controlled), which I believe is more important to speaker longevity than the occasional "cranking them".

I have a pair of Mirage M760s I purchased as demos in 1986. They are still in pristine condition and sound fantastic. 

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.