How many data points are required for one revolution?
@Hustler. Hi. I suggest you use all 48000 vibration samples. As a general rule: In mathematical analysis of measured data it's preferred that you use all available data to estimate some physical quantity. If you perform 48000-point DFTs (discrete Fourier transforms) your first DFT bin frequency will be zero Hz and your DFT bin spacing will be (as jithin said) 1 Hz. Your full spectrum analysis width will be zero –to- 24000 Hz.
When you see a narrow spectral peak in your DFT spectral magnitude samples you can relate the frequency (vibrations/second) of that spectral peak to some gear in your gear box by knowing the gear's rotational velocity and how many teeth the gear has.
If you are forced to use radix-2 FFT (fast Fourier transform) software, I suggest you zero-pad your 48000 samples out to a length of 65536 samples (2^16 samples) and then perform a 65536-point FFT. In that situation the FFT spectral bin spacing will be 48000/65536 = 0.7324 Hz and your full spectrum analysis width will again be zero –to- 24000 Hz.