Digital AGC is a time discrete system. The speed is defined by the required gain range to cover and the amplitude detection dynamic range. If you have to cover 90dB and the measurement is 30dB you need minimum 3 steps to hit the right range and a last step to correct if the detection signal is within the range. If the gain setting or measurement process is some inaccurate you need possible a further step to finalize the AGC. Use a update rate which is integer multiple of the signal periodicy.