As I mentioned in a previous post, there is no overlap between Buddha Sasana of two successive Buddhas in the HUMAN WORLD. Let us see why.
If one attained Anagami stage in the Kassapa Buddha Sasana, he/she would not come back to the kama loka at any time.
If one attained Sakadagami stage in the Kassapa Buddha Sasana, he/she would be born in the deva loka and would not come back to the HUMAN WORLD. He/she may reborn there or at HIGHER realms during the Gotama Buddha Sasana, but would not be reborn in the HUMAN WORLD at any time.
If one attained the Sotapnna stage in the Kassapa Buddha Sasana, he/she could have AT MOST 7 more human bhava in the HUMAN WORLD (at most 7 bhava in any realm too). A human bhava is normally thousands of years, not even millions of years. So, it is clear that one would be out of the human bhava, within at most a few million years. The gap between two successive Buddha Sasana is likely to be many millions of years. Within the past 4000 million years, there have been 4 Buddhas (Buddha Sasana). So, it is more like 1000 million years per Buddha Sasana.