The flood didn't breach its containment on delta halo until the level Quarantine Zone in Halo 2. That would mean that the final message is taking place 2512332 hours (about 287 years) after the events of the games. I believe (for no particular reason) that this amount of time could be connected to how much time the MC is floating in space in the remains of the Dawn. After all, events in a story generally take place in roughly the same time period, so it seams reasonable that our friend Master Chief could be waking up 287 years after the end of halo 3.