Get the Web Timer extension from Opera’s extension catalog.
Web Timer is an Opera port of a Google Chrome extension designed to help you keep track of how you’re using your time online. Nice charts powered by the Google Visualization/Charts API.
You get insight into how you use the web, how focused you are, and which sites are your biggest time wasters.
Run make to package it as an Opera extension and open the resulting webtimer.oex file with Opera. Or drag the config.xml file on top of an Opera window to install it in developer mode.