It's not quite that simple, as the motor will burn out if it needs to run constantly. If the compressor is sized against the airflow of the gun correctly, the tank shouldn't run out and the compressor should at most, run 50% of the time and be off 50% of the time. I have no idea what the maximum run period of the motor should be before it needs to turn off to cool down; that will change from motor to motor. But if the motor starts running and isn't cutting out after five minutes or so (and you can see from the gauge on the compressor if it's pushing the tank pressure up or not), then you are probably using more air than the compressor is designed to provide.

The higher the psi setting on the regulator nearest the gun, the more air you are getting through when you use the gun. So if the compressor is running constantly, try dropping the air pressure setting a bit (if it doesn't affect the spraying). You're getting through double the cfm at 60psi compared to 30 psi, or 50 psi compared to 25psi.

The good thing is that spraying a guitar body doesn't take long, so you are unlikely to damage the compressor motor with a 5 minute spray, even if the motor is running all the time. But if you use the compressor for spraying anything larger so it need to run for longer, or use it for air tools that use a lot of air and you are doing say a big sanding job, then keep an eye on the motor run time and take breaks to give the motor a rest if it looks like it's running for too long.