I know how to create an extent indicator in Layout View, but I have no idea how to create one for Data View. I have 25 bookmarks created for Data View and what I need to do is to display all of them in Data View. I work in ArcMap 10.3.
The ListBookmarks function documentation includes:
ListBookmarks example 3
This sample will convert each bookmark in a map document to a feature.
A copy/paste of that sample with some minor edits to match your environment should enable you to draw the extents of all your bookmarks in Data View (and Layout View).