Given your project is a RPG, you have to consider your game fun factor and balance in combat and pacing of the game's action, adventure, movement, animation and story, system and code complexity. I would say it's hard to say but it's definitely going to be A LOT of time given what I mentioned earlier.
You won't really know how the game looks on screen despite how you know it already is on paper. Some design always change and when design changes time adds up.
The coder will mostly likely want to see your art given he/she needs to add the art to the game.
The game also need testing and fixing bugs, so the time will also adds up more