You can do this with basic JavaScript using setTimeout:
var totalSeconds = 0
function stopwatch() {
// increment the seconds
totalSeconds++;
// display the seconds to the user
document.getElementById("<%=myLabel.ClientID%>").innerHTML = "You have spent " + totalSeconds + " on this page.";
// wait a second and call the timer again
setTimeout("stopwatch()", 1000);
}
// trigger the timer
timer();
Update: If the user is going to be on the page for a while you probably want to display a slightly more user-friendly message then "You have spent 1000 seconds on this page". Here's a quick JavaScript function that'll transform the seconds into time elapsed.