How fast do the blades rotate?

Each different model will rotate at a different speed.
They will be designed to operate at a particular speed to maximise aerodynamic efficiency. If they operate below this speed efficiency drops away VERY quickly (I believe its the function of a cube root).
Typically it will be about 550 rpm, but this will vary significantly and the rate may even need to be set to enable the machine to meet a noise level.

This can be a significant value for a frost fan and it is important to use it correctly.
For instance, one of the requirements under the Marlborough District Council's Proposed Wairau Awatere District Plan requires frost fans to maintain the speed of the blade under the speed of sound (this is to prevent a particularly nasty noise from resulting).
Two resource consents (U70017 and U70190) were granted by the Marlborough District Council where proof that the blades would not exceed the speed of sound was given by reproducing the rpm value of the fan.
This is an inept attempt at providing the required information as there is no means of knowing if the blades will exceed the speed of sound if you are only given the rpm (you would also need to know the blade length and this was not provided). The only sadder part is that the Councilors accepted the values as correct and granted the consent. This is the sort of thing you will have to deal with.