FFT: relation between input data and transform length

Hi, everyone,

For FFT, is there any formula to compute how many input data needed for one output data coming out with respect to the transform length?

I have simulated the situations when transform length are 64 and 1024, I observed that 169 and 2108 clock cycles (input data) are needed for one output data respectively. I don't see any connection between the transform length and data needed for on output so far.