According to data gathered by Bungie, players have spent 235,182 years playing various Halo titles - Halo 2, Halo 3, Halo 3: ODST, and Halo: Reach since 2004. They also say that over 20 billion Halo matches have been played on Xbox Live. Broken down by game titles, Halo 2 accounted for 5.4 billion games, Halo 3 / Halo 3: ODST were played 11.5 billion times (including ODST online co-op), and Halo: Reach matches were played 3.9 billion. Around 108,548,288,596 total kills were recorded across Halo 2, Halo 3, Halo 3: ODST, and Halo: Reach. Players earned 79,027,866,182 medals from the three titles.
All these statistics were a parting gift to the vast Halo community. On March 31, the job of tracking that data was put into the hands of Microsoft. Bungie is officially not responsible for the Halo universe in any way.
You can check out all the statistics at Bungie.net.