StatCounter gives a nice feature: The length of the visits on your site. When visitors leave within 5 seconds, that means they probably weren't interested at all. The rest you can see how long they stayed. How they measure this? They just measure the time between entrebce URL en exit URL. So the details aren't that optimal, but an indication for sure.
For the free StatCounter tool it only measures the last 500 visitis. Not that much for a test. For access to the complete features one has to pay.