You can also calculate time from frequency. If you have a 250hz sine wave, the time to complete one cycle is 0.004 seconds.

t = 1/f

t = 1/250

t = 0.004 seconds

Frequency vs Time:

The frequency is equal to the inverse of the time it takes to complete one cycle. If it takes 0.01 seconds to complete one cycle, the frequency of the sine wave is 100hz.