Set-OrganizationConfig -ShortenEventScopeDefault 2 -DefaultMinutesToReduceLongEventsBy 5
These are set using the Set-OrganizationConfig setting. The three new properties are ShortenEventScopeDefault, DefaultMinutesToReduceShortEventsBy and DefaultMinutesToReduceLongEventsBy. You turn on the reduced length meetings by setting ShortenEventScopeDefault to 0, 1 or 2 where 0 is the default and meetings are not shortened and 1 means to end meetings early and 2 means to start meetings late.
Once ShortenEventScopeDefault is set to 1 or 2 you can set the DefaultMinutesToReduce…EventsBy properties. These are set to the minutes that you want taken off the meeting. Short meetings are those of an hour or less and long meetings exceed one hour. So for example if you set DefaultMinutesToReduceLongEventsBy to 10 then all meetings over 1 hour would be 10 minutes shorter, either at the start or the end (not both) based on the value of ShortenEventScopeDefault.