How long has Halo been in development?
Halo has been in development since around the end of Myth's development, during the winter of 1997. It proceeded through the development of Myth II, briefly interrupted by the final development phase of that game.
At the time of this update in October of 2001, Halo has been in development of some sort or another for nearly four years.
When development began, Halo was slated for a simultaneous release for the Windows and Macintosh platforms sometime in 2000.