Not having followed the thread cannot comment on the coincidence of having 256 samples and also 256 Hz as the sampling frequency.
In any case the general formula below applies to any FFT dataset:
N = 256 # number of samples
f0 = 256 # sampling rate [Hz]
Δt = 1/f0 # sampling period [s]
Δf = f0/N # Δf = 1/(N*Δt), frequency step in [Hz] between FFT samples
NB: the above are physical frequencies, not angular frequencies