Legally I know there's a certain percentage (which is why, for example, the translators of the NIV Bible had to make a certain number of changes so they could separate it from the KJV and copyright it). IANAL however. If in doubt, don't plagiarize?
if you mean so it's not seen as duplicate content by google... no one knows for sure. we do however know that translating to another language and back (maybe even a few times) or replacing a set of words with synonyms generally isn't going to be enough. worry about adding something unique, interesting, and significant to the existing content for the sake of your user, and google will probably treat you just fine.
Personally I'm curious about duplicated content on your own site. Say you have a blog for example. You have one permalink page and one comments page. If the comments page doesn't have any comments, then you'd have two pages nearly exactly the same. A category view would consist of multiple entries, duplicated from elsewhere on the same site. What about that?
There are many insights for this. But in as of my experience its better to have less than 70% similiraty. We use to have more than 80% in our pages, on that time google dosent index our site, even it index it wont show up in the results. So we inserted some dynamic content and pull that down to 68%. Now its doing very good. All my pages are indexed [ More and 300 thousand pages]