Sampling rate vs sampling time of FFT
The sampling time is the time interval between successive samples, also called the sampling interval or the sampling period, and denoted $T$.

The sampling rate is the number of samples per second. It is the reciprocal of the sampling time, i.e. $1/T$, also called the sampling frequency, and denoted $F_s$.

The frequency axis for the FFT is linked to the number $N$ of points in the DFT and the sampling rate $F_s$. It is defined as $f=k\cdot\frac{F_s}{N}$. With $k$ going up to $N$