You can view the scheduled 'Duration' within the 'Presentation Details' tab on the right hand side of the presenter screen. Elapsed running time is also displayed above the main panel.
You will receive 4 minutes of over-run time on the scheduled duration, so be sure to keep your webinar on time. You will be notified when you have entered into your over-run period, and a countdown timer will be displayed. When the over-run timer reaches zero, the webinar will automatically end.