Spark Plug Interval 100cc KPV

When does one know when to replace your spark plug? What is the general rule of thumb in terms of running time?

I would just throw a new one in at the start of each race weekend. Cheap insurance.

Spark plugs are cheap, ergo, every race, keep a spare close by! Their importance/price ratio is very low. I.e. their importance is so high on my list and the plug is so cheap, I don’t run one twice. You’ll get a lot of different opinions.