I stand with my guess of two years after v 5.1 is released (So at least 3 years from now) Kicad 6 development has not yet officially started!!
If there is more money this will simply mean more of the planned features will be implemented There is already talk of putting some nice things for the symbol file format on hold for version 7 as the devs fear they can not be finished till v6 should be released.
v6 will be a lot of busy work where the users see little of it. Rewriting the file format takes a lot of effort and is simply there for preparing for future ideas. Only after the new file format works reliably will it be possible to add new features. (So my personal guess is: kicad 6 will be the file format change without adding new features to the file format. Version 7 will add all the inheritance stuff that gets us lib maintainers excited.)
One good thing that will come with v6 and the new file format is that the whole cache and rescue lib stuff can be finally retired as the symbol data will be included in the schematic file. (so like the pcb side you can then update from library on user action)