Hey @Jon Ostroushko,
I do not know of a way to do this organizationally from exchange.
I have had a similar ask from a client, and found a way to do this through outlook itself. It was fairly complicated, but this post walks you through what needs to happen:
https://blogs.technet.microsoft.com/ewan/2013/11/08/tip-o-the-week-196-change-outlook-meeting-durati...
In short, what it does is change the users options in outlook for default meeting times. So when your user would go to book at meeting at 1pm, rather than having 15/30/45/60 minutes as the option, you could make it 25/55, so the default behavior they are doing would book the meeting for that window.
Note this is not a hard and fast rule, so they could easily just type in the full hour, but it would make the default options what you were looking for.
For my client, this was just to make the change for one user, so I did not have to worry about rolling it out organizationally. But I would think as this is a macro you are doing to outlook, you may be able to roll this out through some sort of group policy. I would think you would also have to include some sort of end user communication letting them know of the change and why (to encourage them using the new time sets).
Hope this helps, I know it is not exactly what you were looking for, but this is as close to it as I know how.
Adam
