GetSpectrumData - what does it fill the samples argument with exactly?

Lets say I call getSpectrumData with a float array of size 1024. According to this question, the length of samples[] is the frequency resolution. So, how do I specify over what period of time/number of samples I want the FFT to be done on? Let's say I call getSpectrumData in Update() and want to get the FFT for the audio that played since the last update (for real-time pitch detection but that's a detail) - how would I specify that I want the FFT to be applied on the last X samples?