I think that there is somewhat like a 10 seconds rule? I don't really know but I think 30 secs are a bit too much in my opinion... Well, I don't know but I also do want to^^
Do not post copyright advice unless you know what you're talking about. ContentID is perfectly capable of matching content shorter than 10 seconds. In addition, there is no hard limit such as a "10 second rule" included in fair use legislation.
Reasons why people dont' get hit for using copyrighted content include:
They got permission.
The content owner didn't upload the material into ContentID.
The original was modified significantly and no longer generates a match.
The video was claimed and is being monetized by the content owner.
The video had a claim which was disputed under fair use and the content owner did not push further.