I was just reading this today: http://www.problogger.net/archives/2012/09/07/how-to-hit-content-scrapers-where-it-hurts/ Don't bother reading it- I will tell you what it says.
It is not much of an article, but there were two excellent points that made sense to me.
Point 1: File a complaint with Adsense if it applies. https://support.google.com/adsense/bin/request.py?contact_type=violation_report&&rd=1 Assuming that these sites are using Adsense, and likely they are, you can file a complaint.
Point 2: File a Google Content Removal Request. http://support.google.com/bin/static.py?hl=en&ts=1114905&page=ts.cs I am not sure how this is differnt than a DMCA complaint, but it should help.
I will add one more point from: http://searchengineland.com/google-scraper-tool-185532
Point 3: Help Google research content scraping sites. https://docs.google.com/forms/d/1Pw1KVOVRyr4a7ezj_6SHghnX1Y6bp1SOVmy60QjkF0Y/viewform Google is not promising to take a site or page down with this option, but it will help Google in the future to tune their algorithm to stop spam sites by giving them a data set to study.
The idea is to hit them where it hurts before filing a DMCA complaint. As well, there is a process for Adsense where all spammers are repeatedly denied once a valid complaint is filed and taken action for.
Now to answer your question.
In the U.S., there is the Fair Use Doctrine that allows quoting and partial use of work in another work if the new work adds value and is related. Spam sites will never qualify for fair use. It does not matter if it is %100 or %1.
In other words, none of it is allowed!
To be fair, we are all used to seeing snippets with links and so on. It had become acceptable that this practice exists despite the fact that it is generally illegal. There is one other legal point I will make later in this response which also makes this an illegal practice.
As well, in the U.S. you can file a civil complaint on the basis of stolen search traffic and ad revenue potential. Not too many people have heard of this. But it exists. It is also hard to prove damages, however, if a site has a history of performance that then suddenly drops as a result of content duplication, then the responsibility is shared by all who violate the copyright. You can file a single civil action against many defendants.
Huffington Post knows this very well as well as Getty Images as both have been sued: HuffPo for snippets with links that outperform the original content as well as cropping out copyright notices from images and reusing them, and Getty because they allowed people to submit and register images that were not the work of the submitter. In addition, the so-called fair use changes when it is a for profit scenario. If someone profits from your work, then you are entitled to compensation automatically.
And as always, a cease and desist letter generally works. You can always hire an attorney and mail a letter and/or send an e-mail to all contacts of a domain name to remove the stolen content or content snippet. In fact, the attorney can simply create a template that you can reuse and save money by doing the work yourself.
Content thieves always will steal content that they can get away with. If they get caught, they often try and avoid problems by appeasing anyone who sends a complaint letter. This does not always work of course.
Try the three points above quickly and file a few complaints before filing a DMCA complaint. Allow Google to research the issue before they actually remove the content with DMCA.
On a personal note: I largely abandoned my websites many years ago because of excessive content theft that drove two PR 8 sites with 10,000 real world page views (not bots) each day down to just a few hundred a day. I blame Google for this because they allowed these sites to succeed in order to make money. I missed out on gobs of cash over a decade. Today, I fight back and there are finally some tools that allow a website owner to take charge. Take advantage of this shift in Google taking responsibility for this industry that they created. It is time that website owners take their own destiny in their own hands.
Another note: Having content scraped generally means that your content is exactly on target. Having your content scraped is a perverted positive sign that your work has value. It sucks that we have to take compliments such as this as a sign of value, but for what it is worth, you are clearly doing a good job!
