While on player screen, press the sleep timer button(bottom right most button). You will be popped up with a menu to set the time in minutes before the app has to close. The + button increases time by 5 minutes and - button decreases by 5 minutes. You can straight away enter the value using Keyboard. The present limit for the sleep timer is 180 minutes(3hrs).

Once you have set the sleep timer, the remaining time will appear in the player screen(bottom left above the control bar).
If you want to cancel an already set sleep timer, press the sleep timer button and press cancel all button on popup.