I would not worry about the life of the piezo. Most are still working on the original squares sold back in 1983. Will they wear out? Yes. I had some older G's, that when I opened them to do a battery change, you could see a wear mark where the spring was in constant contact. But they still worked just fine.

Short answer: It should last decades. The watch will probably wear out before the speaker gives out.

 

I've had some casios from the mid 80s and the piezo works fine. So it should last the life of the watch. I think originally the aim was for a g-shock to last 10 years, but a lot of them last much longer than that. From all the casio watches I've handled I never had one where the speaker wasn't working. It's usually the resin, strap, battery, and buttons that have been issues.

 

and when it's not making sound, it's probably the spring not touch the buzzer, or something wrong with the module / ;-)

 

Thanks a lot everyone for replying.... I will continue to leave them On.... I think its a very cool feature as I am in a profession where I have to monitor my work productivity per hour.