It all depends on what you are attempting to do. For example, if I make an RTP map in engine I can do a 25x30 map in about 30 minutes. For my current game I'm doing parallax mapping with layers over the character, lighting, and collision. Each of these maps takes about 12 hours to make, and about 15-16 hours to finish implementing into the game.
If you just want a basic area where npcs just say 1 line, that takes significantly longer than say making each npc have random speech, choice dialogue, more than 1 line (ie. you talk to them again and they say something else) etc.
For the story will you describe actions in the cutscenes? Will your quests have choices? How many characters will it have? How many classes, abilities, items, enemies will you have?
Unfortunately what you ask is an impossible question, because it strictly depends on what you want to do / add to your game.