Andy tells the office that the show starts at 8pm and runs 1hr 45min (thus ending at 9:45pm). During the first act (while Andy is singing 'Johanna'), Ryan shows Kelly the time is 10:12.
When the play starts, Darryl tells Michael to be quiet during the Overture. Sweeney Todd doesn't have an Overture.
When Michael asks Andy how long the play is, Andy replies an hour and forty five minutes. The approximate running time for Sweeney Todd is three hours.