Only a court can decide if use is fair. As Dale stated, we will know whether it was or was not legally fair when a court decides, and it ceased to be appealed. Unfortunately, there is no formula. Remember that fair use is an exception to rules that make copyright infringement illegal, so it occupies a narrow space practically by definition.
Fortunately this was already decided
(yes, I recognize that this is an old question)
Judge Katherine B. Forrest decided in favor of your favorite YouTuber:
The Court has held that the Klein video constitutes fair use, and further that the Klein video does not infringe plaintiff’s copyrights.
and further that:
But even if this Court held the Klein video is not fair use, the Court would still dismiss Claim II because defendants clearly had a subjective “good faith belief” that their video did not infringe plaintiff’s copyrights. Cf. Lenz, 815 F.3d at 1153.
It is undisputed that defendants understand the concept of fair use and have an
established practice for ensuring their videos make fair use of copyrighted material.
But that answer isn't entirely satisfying, is it? Let's get specific.
Fair use seems to be something that should be clear to both parties: the copyright holders and the user of copyrighted content. While it is true that the percentage of original content may be of concern, there is no specific percentage nominated in Title 17 Chapter 1 Section 107 of the US Code.
In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
It takes vigilance on the part of both parties to cohabit this narrow law. Fortunately, YouTube has recently taken measures to ensure that use that should (presumably in their view) be judged as fair will remain live on their site while a dispute process is pursued. This procedure resolves cases outside of formal legal battles, which can be costly for both parties. However, YouTube has pledged to pay legal fees of content providers that are sued for copyright infringement, in special circumstances (esp. when the use was fair, and when a claimant is abusing, again presumably as determined by some legal counsel at YouTube). They discuss similar matters in their FAQ page.
Why did YouTube let an abusive claimant remove my video?
YouTube takes action to address cases of abuse and misuse in our copyright takedown processes. While we cannot comment on specific cases or our processes, we do look into abuse of our copyright tools and processes, and have a zero-tolerance policy for claimants we've deemed abusive. Misuse of the copyright process (for both takedowns and counter notifications) can result in termination of an account.
And remember that courts love precedent
Future cases will likely leverage the decisions made in the case you mention. And while the following does not have any real legal value, you may find comfort in knowing: there are many YouTubers out there that heavily leverage others' content, and their videos are still up. Again this is not strictly speaking a precedent, but you can safely assume that in cases where a high-profile YouTuber (say CinemaSins, for instance) uses content from many content owners, and uses a great deal of it, they will likely have come up against claims of unfair use, and have emerged with an intact channel. The aforementioned avenues that YouTube provides should be utilized wherever possible to stay out of court.
