Newer cars they have a very long lifespan on plugs.

Plugs "fail" for at least the following reasons:

1) Internal. They open (as-in like a switch) and no longer have a connection between the center electrode and the terminal nut/stud end.
2) They foul, which can be cleaned. You can adjust heat ranges to minimize this potential.
3) They corrode/burn away. This is the visible reason to replace them. Rounded edges on everything don't give a good, reliable, spark. As soon as the edges start getting too rounded over, it's probably time to get a fresh set.

The hotter the spark the longer they last, so any improvement you've done in this regard increases your plug's lifespan. That's also a reason new cars have a longer service life on spark plugs. Back in the day mechanics would just sand blast plugs and back in they'd go. As long as they were clean and made spark, that was good enough. Today, you rarely see this being done, but there is no reason to toss a good plug just because it has reached mile XX,XXX.