I have an older microwave.
when I set it for 30% power on the keypad, it runs for 10 seconds, then is off for 20 seconds. that is using the clock on the microwave.
when I did it for 50% power it was on for 15 seconds and off for 14 seconds. I watched this for 3 minutes as it was heating some water and it was easy to tell when it was running and when it was not.
as I said, mine is an older microwave. the type that you could not reduce voltage to reduce power and that you had to engage and it would take a moment to develop power to output.