How many miles before they need to be changed?

 

I swap copper plugs out every 15k mi...YMMV

FWIW I switched to the Autolite 106's per recommendation of my tuner...

 

Take few out and look them over, to see if it appears to be the correct heat range for your specific build - hot enough to avoid fouling, cold enough to avoid rapid electrode wear. If the gap is starting to open up, they need to be replaced.

There are a lot of good online guides to “reading” spark plugs.