Monster has it posted on their site... The way they spec out the speed it seems more like 150-200rpm.
RPM
What RPMs should I run my mill at?
The best speed to run all of the drill driven mills at is around 150-250 rpms. Does it have to be exactly 200 rpms? NO WAY. A range of 100-300 is fine, and if you are willing to have a little more flour, then you can run it a little faster. I generally tell folks to run it as slowly as your drill will run it without stalling plus a little more. It usually takes a little more torque to get the mill going than to keep it going, so you will have to give it some more power to start the mill, and then slow it down once you are milling. You should experiment with faster/slower speeds and see what it does to your grain.
Motorizing
How can I motorize my mill?
This is a very complicated answer. We recommend you use at least a 1/2 HP motor for our small 1.5" diameter roller mills, and 1 HP for our larger 2" diameter roller mills. Use a capacitor start motor if possible. They have much higher starting torque and should be able to start milling after the hopper is loaded. If you don't have a capacitor start motor, you may just have to have the mill running when you load the hopper. Get the RPMs down as much as possible. To do this you will need to use as slow a motor as you can get, and put a very small pulley on it, and put as large a pulley as possible on the mill. The biggest problem with this approach is that the pulleys end up being very large.
Most AC motors run at the slowest speed of 1725RPMs. This means that you need to reduce the speed by a minimum factor of 10. So you will need a mill pulley that is 10 times the diameter of the pulley on the motor to get down to 172.5 RPMs assuming your motor runs 1725 RPMs. When using pulleys to drive the mill it may be preferable to widen one flat on your mill's drive shaft using a metal file. The flats are designed for drill driving, and will work better with a pulley setscrew if they're wider. This is easily accompished using a metal file with the drive roller held firmly in your bench vice.
Pulleys to complete a motorization project are readily available from several sources including Grainger, and McMaster Carr. Look for a small pulley with a diameter of 1.5" or less, and a large pulley to mount on the mill of at least 10" with 12-14" being preferable. Worth noting is that usually the pulleys will have different pitch diameters depending upon the width of belt you use. If you use a narrower belt, it will ride lower down in the pulley groove, and effectively reduce the diameter of the pulley.
There was a very informative article written in BYO a while back, that has all the specific information you will need to complete your project. We worked directly with the author to provide accurate information. Gear reduction motors are also an option but they are usually out of the price range of most home brewers. If you can find one used for a good price, I have heard of several folks using them sucessfully, but you must make sure you have enough torque to drive the mill, or it will stall while grinding.
