How to determine the parameter of a Complementary Filter?
This is the difference equation for a low pass filter.  Five time constants (5 * 1 time constant) is the time it will take to for the output, to reach 99.33% of the value of the input, from when the input changes from 0 to its final value, and stays there (a step response).  This comes from 1-e^(-5).  As you are specifying time constants, it implies you want a time domain definition for use as a moving average filter, so specify the period of averaging you would like, assume that as 5 time constants and divide by 5 five to get one time constant for use in the equation you have quoted above.