Amplitude modulation means that you change the amplitude. The amplitude is a parameter that goes from 0 (silent) to 1 (as loud as possible). If you apply AM very wildy then you will change the amplitude from 1 to 0 to 1 to 0 to 1 ... If you want to modulate the amplitude less, then you'll go from 1 to 0.5 to 1 to 0.5 to 1 ... Or even less, modulate the amplitude between 1 to 0.9. Than you'll only slightly hear the extra tones.
What do you mean exactly? What do you mean with MS? Do you mean milliseconds? If you want to be able to hear the amplitude go down and up, you'll have to change the amplitude slowly. In that case you can't change the amplitude more than about 20 times per second, because then you'll start hearing the modulation as extra tones. You can set the time of the amplitude modulation by setting the frequency of the oscillator that modulates your amplitude. If you modulate the amplitude with an oscillator at 0.5 Hz, that's a wavelength of 1/0.5= 2 seconds, you'll hear the amplitude go down and back up again in two seconds.
Maybe this patch will clear things up for you. Try changing the depth and the ratio.