How many miles can you usually get out of an HEI coil before it should be changed?? What would be symptoms of one going south??

 

Stock ones in a properly functioning ignition can last hundreds of thousands of miles. If there's other problems with the ignition or you just have a bad one, they can poop out in a matter of a few days. Generally, they either work perfectly or they don't work at all- but not always. I've had them work fine but then "take a rest" after a doing a WOT run. Just shut off and not make sparks until you let them sit for a few minutes. Weird stuff like that.

Best to always have a stock replacement coil laying around for doing "back to back" testing anyway. If you suspect a coil problem now might be a good time to add one to your bag of diagnostic tools. Even if the coil is not at fault you can at least eliminate it from the list of potential causes.