We exactly know that, but we hope it will be atleast long enough to include all monster girls from his encyclopedia wich is also crazy but not impossible, and to be honest a project as big and long as paradox will probably almost never happen again, itsss wayyy to much work and require a lot of people, money, time comsuming, ect.
And since we all know kenko has an obsession with perfectionism for his game he works alone for the most important parts, such as writing, arts, and others, its gonna take alot of time wich i understand, i would do the same tbh.