HFSS solves in frequency domain theoretically unrelated to the frequency scale. So it stacks in the mesh size.
If I am not mistaken, technically it is limited by word precision (supposing they use 64bit processor PFU words) and machine accessible RAM size.
I believe, it is from several Hz to optical region (depending on the problem size, the wavelength number, and acceptable accumulated numeric error noise). We must keep in mind that optics is limited to coherent constant wave model...
Practically, it should work from meter waves to hundred THz on modern workstation clusters.