How to calculate duty cycle of a servo motor?
Speed At 4.8V   0.25 sec / 60°
Speed At 6.0V   0.22 sec / 60°

At 4.8V if it takes 0.25sec for a 60 degrees shift then for a full shift (360 degrees) we will need 6*0.25 = 1.5s which is the period of the shift.
f = 1/T = 1/1.25 = 0.8Hz
The same way for 6.0V
We don't have enough info for the duty cycle of the motor.