First of all, let me show what I mean by a simple example of an FIR-Filter:
I designed an FIR-Filter with a constant group delay of 323 samples and my sampling rate is 500 Hz. Now I can calculate the time delay by this filter with 323/(500 Hz) = 0.646s.
An IIR notch filter has a frequency dependent group delay and I am interested in the time delay around 50 Hz.
We know the group delay is around 2.5 samples at 50 Hz and we sample with 500 Hz. Can I do the same calculation as before? This means the time delay for 50 Hz is 2.5 samples/500 Hz = 5ms.
I am not sure but I think the correct calculation is with fs/2, that means it's 2.5/250 = 10ms.