How often should you change spark plugs?

If the fuel system is close (reading a plug)not running a data logger.What would be a rule of thumb to change spark plugs.Can you read a plug after the second or third pass and be correct seems like the second pass will change the plug reading and be a little confusing and the third pass it's way out there! Just a thought I would be interested in others opinions. Thanks

 

If you want to keep accurate tabs on your fuel system run to run you should use new plugs every run. It's in your engines best interest to read the plugs so you know whats going on in there.