Instead of asking this question I would encourage you to look at websites that are ranking well in Google. You will notice that keyword density does not correlate very well with rankings.
As for determining what is enough for ranking that depends greatly on the specific situation. If you are trying to rank for a non-competitive keyword you will find it is alot easier than ranking for a profitable keyword.
IMHO keyword density is mostly an out dated concept. I would encourage webmasters to be concerned about writing content engages users. Good content will naturally have sufficient keyword density, generate positive usage signals and help to convince webmasters that your site is worthy of a link.