I like the different types of visitors I have – those who want to make a fast stop to find what they want – get them in, make sure they see exactly what they came for, get them to click and leave. Those spend from a few seconds to 3 minutes or so.
Then there are those who don’t know exactly what they want and spend some time looking at options or getting some info – many of these are repeat visitors and spend from a couple minutes to 15 minutes.
The others are loyal repeat visitors who spend time exploring and being entertained. They spend a bunch of time, sometimes they spend hours.
Each type of visitor has their own value. Many sites target one or the other – the most popular being the get in, click, get out type. Another business model is to build communities with message boards. All approaches have their merits.
So there isn’t any one visitor time frame that is better than the other, it all depends on what your goals are. My site is old and it’s unusually broad in coverage. It just kind of has a life of its own. 
The counts above can give you a better idea of what type of visitor you are attracting and let you cater to them better.