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.