Depends on the length of the article. I would stick around 3 to 6% to keep the search engines happy. You can get away with around 10% and do very well with yahoo. But Google wont be as forgiving with this.
If you want to use the article for the website, then the keyword density should be 2-4%. if you want to submit your article to article directories then the keyword density should be 5-7%.
Keep your keyword density at lowest level possible. I have read about it in many places here in DP that many preferred 2-3% max keyword density on any articles.
i think 2-3% keyword desnity is enough as far as i experience with ezines articles and i never like to write more than 500 words articles because the more long your article chances that your ctr ratio would be down .
Forget about everything above. just write the article on-topic and naturally so that it includes also topic-related phrases. Gone are the times when just repeating your main phrase 4 times across the article was enough...
Just write naturally with the topic you are working on, it will just come naturally, avoid repeating phrases over and over again.