When adding a booking using the 'ADD BOOKING' button in the Diary/Floorplan or Run Sheet page, upon selecting a booking type, the duration will auto-fill with the default Max Duration set up in the settings for that booking type. When the number of guests is entered, the duration will then dynamically change to reflect if "Rules dependent on number of guests" that been set up in the booking type.

E.g When adding a ‘Dinner’ booking, the max duration setup in the Availability section in the settings is set to 1 hour and so will auto-fill this time as per the screenshot below:  

If ‘Rules dependent on number of guests’ have been set for that booking type to give 1 - 4 guests 1hr 30mins duration - it will then automatically change to reflect this.