As a flight instructor I admit I made a mistake early on teaching students about "rotation speed". As a result, they would stare at the ASI instead of looking out the window. Truth is, airplanes fly when they are ready, providing you place them in the correct attitude. I suspect lift off is at a consistent A/S. However, rotation should begin as soon as you apply power. That is, you should be gradually applying back pressure as speed increases, until the point of lift off. So technically, rotation speed is ZERO.
Sorry to be "teaching" you probably already know this. You probably want to know an exact number for another reason. A good rule of thumb for rotation speed is about 1.1 of stall speed in clean configuration.
The feds say not less than VS1.
SFAR Sec. 23.51 - Takeoff speeds.
(a) For normal, utility, and acrobatic category airplanes, rotation speed, VR, is the speed at which the pilot makes a control input, with the intention of lifting the airplane out of contact with the runway or water surface.
(2) For single-engine landplanes, VR, must not be less than VS1.