Click the timer/clock icon to the left of the play button. Once the timer icon is green, double click to adjust the timer to your desired focus session length. You may enter an amount of time from 1-240 minutes.
The timer will return to the set time of your session once the session is completed. If you edit the timer during a session, it will stop that session and allow you to enter a new time, or start a new session with the previously saved time amount.