Mainly I think that it depends on how well-formed the piece is in your mind before you write. If, in a sudden bout of inspiration, the idea justs come to you, fully formed, then much less time will be needed in order to polish and/or complete it. On the other hand, the opposite holds true for those who have a general idea what they want to write about but have to struggle to find the specific process or words to express it.
For according to the trollish philosopher Plateau, "if you wants to understan' an enemy, you gotta walk a mile in his shoes. Den, if he's still your enemy, at least you're a mile away and he's got no shoes." — A Blink of the Screen by Terry Pratchett
