We have a pond on our property, just up the hill from our chickens for our ducks. We keep them penned until they are about 2 months old to ensure they are too big for turtles and large fish to make a snack out of. Then when they're old enough we take them up, toss them in and that's that. Our only problem is that the pond is not enclosed 100% so that our horses have access to it for water so they have the option of coming back down to the chicken pens where they may also meet with foul play if they wonder too close to the coon dogs. They are almost completely dependant on me for their feed and expect it daily. I've tried skipping a day so as to "wean" them off of feed being brought to them but they just get out of the pond and head down to the chicken pens where the feed building is and as soon as they see me, I'm swarmed with not so happy birds. lol Since going to the pond a week ago, we've lost one to the dogs but have otherwise had no problems. There's also a large female goose that has been a resident to the pond for several years that acts as a bit of a mentor for the little ones and helps keep them in line. When they wonder out of the pond, I grab some feed and make them follow me back to the pond before being fed. I'm hoping they'll learn that the food will be given at the pond and no where else so they'd be just as well off to stay in it. But the goose and her now deceased grown drake duck companion never stayed at the pond 100% of the time either.
In my opinion, I'd keep them up for about two months, then turn them loose into the pond. But you'll still have to give them some food every day.