That would be a very difficult metric to validate, even if you could find that information which I don't think you can outside of the BES.

For example, I could start the browser and open a page, then put the browser in the background for several hours before coming back to it later without ever closing it. In fact the Blackberry browser never exits. The most you can do is go back to the start page. This is because one of the things the Blackberry browser does is listen for push data.

The BES can tell you when a particular page was loaded, but that is about it.