@kglad
I actually changed the settings for Start to 00:00:00 and Duration at 00:05:00....so essentially a 5 minute loop. The file exported, but unfortunately the file ran it's simple animation (about 15 seconds) and the remaining 5 minutes of the movie was the last frame, so no loop. Did I miss a step?
Typing a time span in the export window won't make it loop. Here's 2 things you can do:
1) You could nest the animation inside a graphic symbol (select layers > right click> convert layers to symbol) and extend the frames in the timeline to make it loop for the desired duration, which you can then export to video (without span values).
2) Export the 15 seconds of animation as is. Assuming you're uploading this to social media there should be a loop option on the website itself.