What are the different methods converting time from samples to seconds?

Can anyone help me by providing the different methods/formulas of converting time from samples to seconds, where for example the sampling frequency Fs=500.

[Guillaume]

It's really not clear what you're asking. For a start "converting time from samples to seconds" is meaningless. samples is not a unit of time.

Calculating the sampling duration of a number of samples when the sampling frequency is Fs is trivial. There's only one formula:

duration = nsamples / Fs;

Surely, that's not what you're asking?