Is there a correlation between the reveal of a generation's last mythical and the release of the next generation?

Eg:
When was Volcanion revealed, released, and when did Gen 7 release?
When was Genesect revealed, released, and hen did X/Y release?
And so on for Mew/Celebi/Deoxys/etc.

With Zeraora still not revealed, can we imply from that a minimum time before Gen 8 is released?