Halo 3 has been played for over 154,109 years.

[JaggedSac]

During M$'s CES keynote it was said that Halo 3 players average 150 hours of play.  At 9 million sold, that means:

150 * 9,000,000 = 1,350,000,000 hours

1,350,000,000/24 = 56,250,000 days

56,250,000/365 = 154,109.6 years