The legality of clips on xxx derives from the Fair Use statute, which is part of copyright law. There has been lots of misinformation over the years about what is regarded as fair use. Contrary to what you may have heard, there is no "rule" based on percentages or time limits. The principle that we encourage our users to observe when posting to xxx is that you should only post as much of a given work as you need in order to make your point.
The key to making these posting fair is to significantly transform the clip from its original context and to add cultural value, such as is often the case for educational re-uses. I can't, however, advise you on the legality of any particular posting -- this is your responsibility to decide in good faith. The fact that you are asking these questions and concerned about contributing responsibly is an excellent start!
The best resources for thinking through fair use that I know of are those created by the Center for Social Media -- of particular relevance here may be their guide to best practices in fair use for online video:
http://www.