courant limit
formula given by you is the CFL stability condtion for FDTD.
this is the courant factor???????????
You do like question marks don't you? (Here are some more: ????)
In any case I misunderstood your question since I could not believe anybody asking for the
Courant factor.
Basically you have 3 conditions to satisfy:
1) the Courant limit: dt <= CFL(dx,dy,dz,epsilon,mu) to make sure your simulation is stable
2) some similar condition for high conductivity materials (which is usually ignored by most people
and also depends on the discretization method)
3) a time step which is small enough to allow fine enough resolution of the shortest wavelength
of interest.
If your geometry is small compared to the wavelength you can ignore 3 and therefore anything
satisfying 1 is ok.
If your wavelengths are small then it depends on
a) the algorithm
b) the desired accuracy
For b) look at Taflove and 'numerical dispersion'
For a) standard FDTD => usually lambda/10-20 depending on your model.
(If you have very long models and phase matters, even lambda/20 may not be good enough)
For low dispersion FDTDs (NSFDTD/Forgy's FDTD etc) lambda/3-4 may be good enough
Summary:
Without knowing details about your model no-one can give you a detailed answer.
If it is actually 3) you are worried about don't ask about the 'Courant factor' ask about
numerical dispersion.