We quote one run-time for each mayor element of each Murder Mystery, but this is at best an approximation, since run-time depends on the nature of the production, the slickness of the cast, and, especially, the volatility of the audience!
For scripted elements, our estimate is based on the normal rate of spoken English (six minutes of stage time per thousand words). Generally this equates to a reasonably slick production of the text on the page.
For unscripted elements - improvisation and audience interrogation - we rely on estimates from the author. These are largely on the basis of original productions of the mystery, but they will vary enormous according to staging.
One useful piece of advice for productions based around a sit-down meal is to work-out the section run-times in advance (by rehearsal) and discuss these carefully with the catering staff.
(Disagreements with chefs have created a whole sub-genre of murder mysteries.)