
Some of the most enjoyable and well developed NPC romance mods for BG2 alone have taken (and still are taking) years to complete, so I don't even want to think about how long it will take to make all the areas, quests, NPC specific quests, if any, and hopefully NPC romances. Even if you get help making NPC's for your mod, making their characters, banters, quests, interjections, romance talks and flirts, if any, and items will take a huge amount of time by themselves.
But again if you're serious about finishing this then kudos to you, just make sure you really know what you're getting yourself into.