Caution!! When Revit's little timer goes off and says you need to Save it opens the Save Reminder dialog. If you also happen to decide to use your scroll wheel to zoom in or out you may have noticed that Revit has moved its focus to the dialog and the Save Reminder Interval List Box. Scroll causes the reminder value to change. If you think you have it set to every 15 Minutes but check it and find something else, chances are good that you bumped into this issue.
Beware the save reminder and your scroll wheel!