Plagiarism is the copying of ideas, text, data and other creative work (e.g. tables, figures and graphs) and presenting it as original research without proper citation. There are no legal rules permitting the use of a specific number of words, a certain number of musical notes, or percentage of a work. Whether a particular use qualifies as fair use depends on all the circumstances.
Indiana University (https://www.indiana.edu/~istd/examples.html) defined the word-for-word plagiarism as “when a writer takes a sequence of 7 or more words from another source, but fails to identify the quoted passage, fails to provide the full in-text citation crediting the author(s), and fails to provide the bibliographic reference.” We define the “word-for-word plagiarism” as the author(s) takes a sequence of 6 or more words from another source without appropriate citation.
Self- plagiarism: “Authors who submit a manuscript for publication containing data, reviews, conclusions, etc., that have already been disseminated in some significant manner (e.g., published as an article in another journal, presented at a conference, posted on the internet) must clearly indicate to the editors and readers the nature of the previous dissemination.” (http://ori.hhs.gov/images/ddblock/plagiarism.pdf )
To avoid plagiarism, you may paraphrase, i.e. transfer original text into your own words.