I beat it in one day on Normal. Like I would guess 4-6 hours depending if you do the audio logs. I would recommend this map if you do. The audio log story is pretty interesting...There is a video attached to each location so you see exactly how to get it. http://achievementhunter.com/games/halo3odst/guide
Also I do believe they have to be gathered in a certain order. I spent countless hours looking for missing ones. When I did it in the order listed I had no problems. Good luck!