Google has not released any information regarding acceptable keyword density. But, the accepted theory is that your maximum keyword density is 5%. That means if you have 200 words on your landing page, no more than 10 of them should be the keyword. Five percent is a high density and it's difficult to exceed that mark.

I generally look for a keyword density anywhere between 2% and 5%. That range is assumed to be good for Google SEO and Google Adwords.

I tend to make sure my keyword appears...

1. in the page url
2. in the page title
3. in the meta description and meta keyword tags
4. in the h1 tags

If I've done all four of the those, I find that I can have naturally written content without concern for keyword density and still get good quality scores.