Leaving your browser open on a page doesn't cause data to be sent to that page constantly. It's only sent when you load the page, and then SMF keeps a record of it for 15 minutes before treating it as though your session is over and your next visit won't count the time taken between page loads.
You could get a Firefox extension called ReloadEvery and tell the page to reload every 4 minutes, but that's just a reason why the time spent stat is unreliable.