Select the helicopter blades as a group, picking the group whose pivot point allows the blades to spin in the proper direction. Confirm the pivot point is located in the center where the blades attach. Adjust it to the center if necessary. Set a keyframe at zero.
Move the timeslider to the end of your animation sequence. Rotate the blades several 360-degree cycles. Set a new keyframe.
Render the animation at 30 frames per second to confirm the blades are spinning at the desired speed. Adjust the number of 360 rotations until you feel happy with the rotation speed. Override the second keyframe by setting a new one with the adjusted rotation. Repeat this process until you have the desired effect.