The dimensions of a microwave's Faraday cage are usually a multiple of the wavelength of its cavity magnetron. The interior is bathed in microwave-frequency electromagnetic radiation, which has a smaller wavelength than either visible light or infrared. Standing waves build up inside, with the most intense action happening at certain fixed points, as illustrated in the animation below. After the marshmallows melt, distances between these points can be measured with a ruler. From there, frequency = velocity/wavelength. The answer is going to be of the form "per second," otherwise known as hertz.