How do I calculate the time interval if I know the frequency?

Frequency is expressed in Hz (Frequency = cycles/seconds).
To calculate the time interval of a known frequency, simply divide 1 by the frequency (e.g. a frequency of 100 Hz  has a time interval of 1/(100 Hz) = 0.01 seconds; 500 Hz = 1/(500Hz) = 0.002 seconds, etc.)