How long can a video clip be id used educationally before it violates copyright?

Answer 1

There is no predetermined amount of copyrighted material that can be used before an infringement occurs so yes even a few seconds of a copyrighted work could be problematic. Each "fair use" is established only on a case by case basis.

US Copyright Law sets out criteria that has to be considered before something can be judged to be "fair use"

# The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes # The nature of the copyrighted work # The amount and substantiality of the portion used in relation to the copyrighted work as a whole # The effect of the use upon the potential market for, or value of, the copyrighted work