Just creation of creatures and npc characters can easily take over 6 years for today’s quality standards if the team is small.
You can only achieve such project scale in short time if you already have a library of assets that you can reuse, like Squareenix does for the non flagship mobile titles. For new console release it’s at least 4 years of work on 3d assets and they have hundreds of artists.
In FF15 there’s 13 years of work on ‘new’ 3D assets that could fit the game style. If made by one single artist that would translate to over 1300 years of work.