I'm not sure what it is for ecommerce sites, but I've always thought for content sites if it's 60% or less you're doing really good.
Not sure what others have found.
It has seemed to me recently that people are less and less inclined to actually bookmark sites and go directly to them, even if they like them. Which is why I wonder how relevant it really is anymore to worry about that ratio at all. It used to mean a lot, but does it still?