I hope you have some experience raising poultry/fowl.... 800 is no small task.
Stocking rates for ducks and geese vary depending on how you pasture them, moveable pens, free range etc, what quality of grass/legume you have, rainfall etc. Ducks enjoy young tender grass, so you will have to clip it for them to forage well. You will also need to reseed it every year most likely as ducks are hard on pasture (they eat it down fast), so that's a cost you need to factor in as well. In addition you will be feeding them supplemental feed as well, probably 50-70% of their diet will come from purchased feed.
In my experience (and more aptly a friend of mine who raises thousands of ducks) 50 per acre is a good place to start. You'll have to move them multiple times per day to keep them on fresh grass.
Resting period is similar to that of any other rotational grazing system, 6-8 weeks, also depends on regrowth of your pastures etc.
Also, you have to consider processing all those ducks. They are hard to pluck yourself w/out proper equipment, and a processor will charge you extra to do game birds, around here it is $3-5 per duck to process compared to $1.10-$1.50 for chickens.