How to extend the duration of a layer

Is your 30-minute music video one shot? If it contained multiple shots, it would have been much more efficient to create the individual shots in After Effects and edit the final piece in Premiere Pro. It would also be more efficient to add an overlay in Premiere Pro.

To tell if you have a video that will loop seamlessly, the first and last frames should be identical or one frame from identical if there is motion in the shot. If the video passes that test, you can go to Interpret Footage in After Effects using the File Menu, choose Loop, and select the number of times it will loop.

If you want to loop a video in a timeline, you need to find the first frame you want to use, set the layer in point, then move down the timeline until the first frame repeats and is identical. You can check that with the screenshot camera in After Effects.

When you have marked that point, you move back one frame, set an outPoint for the layer, Pre-compose moving all attributes to the new comp, and trim the new comp to the length of the layer. 

Then you enable time remapping on the nested comp (your pre-comp), move to the last keyframe, then move back one frame, so you see the video, then add a new keyframe using the diamond in the keyframe navigation tool in the timeline. Then you delete the last time remapping keyframe (after the last frame has played) and add loopOut() as an expression to time remapping. The nested comp (pre-comp) will now loop for as long as the layer is, and you can make the layer as long as the comp.

A lot of this workflow is covered in different sections of the User Guide. Anyone new to After Effects should spend several hours learning the workflows and UI basics. AE is an incredibly complex application; it takes most people about 100 hours to understand the workflows and tools available. I have been using After Effects for almost 30 years, and I still refer to the User Guide and the Help files often. There is always more to learn, and there are always more efficient ways to do what needs to be done.

Set the repeats in the footage interpretation. Right-click on the asset in the project window et voila! You may want to read the online help on some of these basics.

Mylenium