I believe some humans on earth currently possess "UFO" type ships with antigravity. I don't think they will voluntarily share this tech... I think it will be after the real world war 3, after the human population is brought down significantly before that tech is shared with ALL of humanity, albeit a much smaller humanity. If you really think about it, humanity as it is, is too irresponsible for that tech.

All that said, I give it 200-400 years before bebop