5 minute visits are typically on the more transactional pages - they are looking through listings picking options, looking at photos, etc. before making a decision or leaving. It seems a good amount of time on site to me, but I don't think it's much more than average in that category. I think Google has a pretty good idea of how quick is too quick for a bounce back to the search results and I imagine that number varies for each query (or category of queries). I know from speaking to a friend who is a former Bing engineer that this kind of logic was definitely being worked on and implemented over there as he was the one doing it. I don't see any reason that G wouldn't do the same.
But of course, trying to improve the time-on-site / bounce-rate stats is really quite tricky to achieve for some terms. I get traffic on some pretty random phrases where we clearly are not the type of site they are after. Naturally these have a high bounce rate. I wonder whether I should manually noindex these somewhat non-ideal forum discussions to avoid them slanting things.