I suppose it depends on the device?- an LED might get to full brightness in a much shorter time than a device with inertia like a motor would get to full speed.Just curious.....
I think you answered your own question
If I hook a motor up to a battery through a manual switch and switch it off and on every 30 seconds, in theory that's PWM but too slow to fool the motor. The motor will run for 30 seconds at whatever speed it gets to for that battery, then go off for 30 seconds.
but, you have to admit, the average RPM over that time will reflect the supply duty cycle