Depends on the plot. If it is like Morrowind, then limitless. I usually don't have a length, allowing the player to explore the world and do jobs after the main mission.
Depends on your story. Usually, I have a low EXP rate from monsters, and a high EXP rate from quests and sub-quests. I never give monsters items, just gold. Items must be found in a chest or at the shop.
Well, as many as it takes to make a fully realistic world. I often find myself making a place the player might never visit, but it is there cause the area is still accessible. All areas in the vicinity of the main quest should be accessible.