When it comes to video, copyright law suggests a few things:
1. Only use up to 10% of the total item or three minutes, whichever is less. This means that you could clip about three minutes (or 10%) of a video that you do not own the copyright to.
2. For music, copyright law recommends up to 10% of the total work but no more than 30 seconds of the music or lyrics from an individual musical work.