My thought about spark plugs is that they are like factory workers.
You replace them only as necessary - ie they don't do their job.
Now, with as conservative as the rules are for the 206, you can foul the 3910X (low temps, low ignition, rich fuel mixture, etc.) Replace it when it becomes suspect of not doing it's job - ie fouls or misfires. You also need to keep check on the gap so that carbon doesn't build up and cause you to be illegal in some sanctioning groups.
That said, I know folks who replace them every race day, whether they need to or not, just as a preemptive measure.
We certainly sell a lot more plugs than fail, that's for sure.