How long do good speakers last?

Tex, they are Italian. Ferraris last a very long time. If you don't drive them.
At any rate it is a little late to worry about this now. They are great sounding speakers. Good luck!

Keep them out of the sun. Should last 20 years or so before you need to replace surrounds if you don't abuse them. Amplifier clipping will damage tweeters.

Good to see so many people looking after their gear.

My Walsh 4's were made in 1984 and re-foamed in 1998.
Currently being re-foamed for a 2nd time after 23 years of service.
I was advised by OHM that they're actually in pretty good shape but they're being re-foamed as preventative maintenance and to offset shipping costs.

They get lots of good clean power from my NAK PA-7A MKII amp.
That helps longevity also.

Totally dependent on materials and environment. 

Good speakers last until they're not good any more.