I did some research into this some time back and there is no specific percentage - it's a myth. Check out this article on keyword density.
Use 1000 words articles! And make the density 1.8. 1000 words articles are more more effective if you compare with 400/500 words articles.
Keyword density is a number of times a keyword or a keyword phrase, in proportion with other words appears on a Web page. The more times the keyword appears in relation to the total number of on page words the greater the overall keyword density.
3% Density is the best ... you can take it to 4% too .. but best practice is to keep it within 3% ....