as you know, the time required for a given blade to reach 63.2% of its maximum rated speed in a no-load condition should be the definition of TC . so, I think first you should find a manual for such a rotor which contains some information about the cutoff frequency, in this regards, I think it should be something in order 10^-5
Significantly very fast. Standing at the control panel of a Vestas 850 kw , I saw the display was showing the instantaneous power production and pitch angle. The display was updating either every second or half second and each reading gave different readings. so a time constant for the pitch of under a second, for 40 m blades is astonishing!