That's the problem with a stateless environment... you don't know exactly when the user leaves your site. Once the user has the HTML in his web browser, the connection between the client and the server is terminated. (This is the definition of "stateless")
However, what you might be able to do is write code in the global.asax file's Session_Start event to store a time value for each session and then calculate the elapsed time in the Session_End event. This would not produce accurate times though. But you may be able to get rough estimates. (remember sessions don't time out until 20 minutes, by default)
Good luck.
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