Click and hold the timer icon. That will bring up the Custom Timer Dialog box for you to enter the amount of time from 1-240 minutes that you want to stay focused.
If you would like to do an un-timed session, you click the timer button once to return it the to clock icon (no numbers will be displayed). Then click play and you will enter an un-timed session. Clicking the timer icon again to turn it red will reactivate it.