OOC - World Building - How much is too much?

I think you would be hard pressed to find an RPer who doesn't like world building. Creating your own unique world, all the way down to the details of miniscule proportion, can be an addictive pass time.

If you are designing a world for your game, how much world building do you do before you start the game? Is there too much and is there too little?

And do you let players add to your world, or do you prefer to create the world for the players, guiding them into new areas you have created.

And players, how do you feel about world building within someone's game? Any apprehensions or questions?