On late model engines:
On Iridium tipped plugs, I'd go by mileage. Reason is that there's a tiny
hard to see iridium pad on the ground electrode that's super thin, when it burns
away at about 100K miles, the gap will look OK and even gage OK, but as the iridium
disappears off the ground, you'll suddenly burn away the plug to a misfire and damage
the Cats. The fuel mixture is lean and hot so the soft steel in the ground electrode
won't take it.