The content required in order for the Blue Orb to not display varies based on the screen resolution and the size of the user's browser window or device screen, there is no standard for how much content is needed in order for the blue orb to not display.

For example, if you only have 2 Items displaying in Recent Activity. This content does not fill the browser window on a computer, and the loading icon (blue orb) will display infinitely.

If you shrink down your browser window, or click the 2 items in your Recent Activity to expand them, then the blue orb stops displaying, and you get the following message:
You have read all the content in your activity stream.