The exact match links, I usually don't have more than 1%. But when combined with all the partial match I've usually been doing around 15% safely.
Sites I'm building at the moment though, I'm dropping that down to 10% just to be on the safe side for future updates. Google is probably going to keep on refining its filters for over optimization, so I want to be on the safe side.
I do believe that the % you can get away with does vary for different search terms though.