Do I need the sampling frequency in order to gain periods out of FFT?
This is DSP – everything that defines time or frequency is only measurable with respect to your sample period; so without that, your FFT output is but a relative measurement.

In short: If you can live with giving information like "events per sample period", then you're fine, if you need to say something about "per hour" or so, well, that information has to come from somewhere, and if you're not feeding in that information into your system, it can't have it.