I got asked today to provide a benchmark value for time on site for a competitor of ours. They asked me if I thought that 3 minutes and 41 seconds was a good time for someone to be on their site. Not an easy one to answer. Users are here for a good time, not a long time. Get them to where you want them. Quickly. Then cross-promote them. Engage them. Then the good time can turn into a long time. But Time on Site certainly won't tell you.