Is this to resolve a very strong resonance or just to get transmission/reflection at many frequency points? Autoshutoff min is to ensure the accuracy of Fourier transform. Frequency resolution can have two different meanings:
1: you want accurate spectral result, such as transmission, reflection. In such case, you do not need many frequency points, no MHz frequency resolution is required for photonic simulation, since any other frequency result can be interpolated from coarse sampling. Since it takes more time with more frequency points for a given bandwidth, in general 50 points are sufficient, except that result is wavy.
2: You want to get the resonance spectrum with very high Q. eg, Q is on the order of 10^6 or above. In such case depending on the result you want, the simulation time can be long or short: