From my testings the 30% minimum level that is often recommended is NOT enough anymore.
You should aim to have at least 50% uniqueness. Search engines are getting smarter and can spot spun content very easily if you only replace words with synonyms.
For example:
I have {bought|found|purchased} {some good|very good|excellent|tasty} {fruits|apples|oranges|vegetables|potatoes} {today|yesterday|this morning}.
This kind of spun article is not going to give any good result for several reasons:
1/ The resulting articles will have the exact same number of sentences
2/ Each sentence will have the same meaning (synonyms are known by the search engines)
3/ Each sentence will have approximately the same number of words
It's very easy to detect this, even for a bad programmer

And this is how the majority of people is doing it (and the reason why so many say that spinning article doesn't work).
Now you have to spin at the sentence level + at the paragraph level to get long lasting results.
Actually the further you go from the individual words for your spinning the more powerful it is.
Look:
If you spin by words, the overall structure of the article is exactly the same, same sentences except synonyms that are easy to detect.
If you spin by sentences, the structure of the article is still the same but the length of the sentences are very different, and we're not using synonyms that are so easy to detect, instead we reformulate the meaning of the sentence and the grammatical structure of the sentence itself is different. Spinning by sentence is > spinning words.
If you spin by paragraphs, the structure of the article becomes different - it's becoming very difficult for a programmer to find that both variations of a same paragraph are similar. The number of sentences is different, the organisation of the sentences is different.
If you spin by articles, you have the best uniqueness level and this is exactly like writing different articles. You see this is why I said the further you go from the individual words the more unique it is.
Vita Vee