Helicopter blades spin from 120-600rpm depending upon the size of chopper, length of blade, number of blades, and supersonic tip characteristics.
I'm led to believe an Apache at hover is spinning around 400-450 rpm.
Shutter speeds around the same value would need to be used.
One thing to remember is that you don't need to match the rpm with your frame rate, you just need to match the time of one blade moving into the position of the blade in front of it. For instance, you have a two blade fan with a rotation of 100 rpm, you only need to shoot 1/2 the rpm, not the whole revolution. With a four blade, whole, 1/2, 1/4, 3/4 of rpm. the same with five, six, or whatever blades.
You just need to do some math fractions and figure accordingly.
So if you are shooting a two wing/blade jet ranger revving @ 60 rpm, you can freeze the blade action at 1/60 or 1/30 fps. As someone else has said, you will probably need to test shoot like crazy to get the rate dead on & cross your fingers that all the variables remain consistent.