A click is produced when a very fast level change in the audio signal occurs. You can easily check that on your home stereo when you play back a CD and switch the Source Selector back and forth between CD and a source that doesnt play anything. The brightness of the click depends on the speed of the level change. The faster the level changes, the brighter is the click. So, the level change speed can be compared with the cutoff of a lowpass filter.
There is an easy formula for it: Lets consider a level change from full to zero or from zero to full output from one sample to another on a machine that uses 44.1kHz sample rate.
So, we first transfer the sample to milli seconds:
1 sample equals 1/44100 second, which is = 0.02267573696ms. To calculate the cutoff frequency of the click, just use this formula: Cutoff Hz = 1000 / Level Change Time ms
which in the example results in: 44100Hz = 1000 / 0.02267573696ms