The time period of a simple pendulum depends upon: 

The time period of a pendulum is the time taken to complete one oscillation.

When measurements of the time period are made while varying factors such as the mass of the bob, length of string, the amplitude of the pendulum, and density of the bob, it is found that:

• The time period of the pendulum does not change if the mass, density or amplitude is changed.
• The time period of the pendulum changes when the string of the length is changed.

Therefore, it can be concluded that the time period of a pendulum depends on the length of the string of the pendulum.