Well... that's a more complicated question than you might think. I'd say it varies a lot, depending on the content of a site and of particular pages. For example, if I ran a site that showed the current weather and people clicked through to my site from the SERP for a query like [current temperature boston ma], I'd expect a very high bounce rate, because users would see the information they wanted immediately and then they'd be done.
But when you talk about a consistent bounce rate for an entire site, where some pages will have a higher rate and some will have a lower one, 15% feels really low to me. You have to figure that this means that very few pages have bounce rates over 70%, or the average would be quite a bit higher, and there are going to be some pages where the rate is well under 10%.
And since I don't know anything about DiKaKo's site, my observance that 15% feels low may not be realistic, but in my experience with a wide variety of sites in a wide variety of niches, I've seen a rate that low for individual pages, but I don't think I've ever seen it as the average for a whole site.
So what's a good bounce rate? It's one that's lower than it was a few months ago. It's one that went down after you made a change to your page's layout, or added new content. It's one that didn't go up after you increased your click-through rate. It's like what that judge said when he tried to define "obscenity": I can't tell you what it is, but I know it when I see it.