Using steel shot, I'd say your max in that configuration is about 30 yards for clean, ethical kills. I use #2 for puddle ducks, #3 for divers. If it is early teal season, I use #4 or #5. #6 is lighter than I would feel comfortable using on ducks. With #2's in 3" out of my Benelli, I feel good out to about 45 yards or so. I do my best not to attempt shots outside of 50 yards, unless I have a good degree of certainty I'll make a kill. (ie. very little wind, usually head on shots, little leading the duck type shots)
If you're using a lead substitute shot, you can probably stretch those distances about 10-15 yards.
I hunt public land, but the pool of duckhunters in my AO is pretty small, so I don't have many issues with others encroaching on my spot. As others have said however, you want to be completely setup about 20-30 minutes before legal shooting time (or sooner depending on setup spot pressure from other hunters). I've grew up hunting the Eastern Shore of Maryland, and have duck hunted in SW Montana lakes and rivers, Arkansas, Mississippi, Louisiana, and Alberta. The one thing all of these have in common? 80% of the activity was between legal shooting light and about 0800.