spoke wear and maintenance
 

Do you or should you ever replace the spokes at a specific mileage, due to the stresses when riding even though if they havent started to break?

Not that i've ever heard of, I'm still riding on the original wheels on my '89 Dawes Galaxy, and they are still true, and I put them through quite a bit of stress (on road though).

I'm not sure about on a MTB though.

No, if the wheel was properly built with good quality spokes, they should outlast the rim.

Except for a mid-80's bike with low-quality spokes (which lasted 8000+ miles anyway), I've never had a DT or Wheelsmith spoke break before the rim wore out from brake track abraison.