What you need to do is change the web page that you're browsing to. If you can't do that, you can stop reading now. If you CAN, try this:
Add code like this to your page:
Sys.Application.add_load(function() { anotherFunctionCall(); });
Combine the functionality of that method with a flag indicating whether or not the web page and the ajax stuff have finished loading, store the time in a database, and use a web service to retrieve that value.