I have two golden retrievers that I do trust with my chickens unsupervised. My younger female took much longer for me to trust than my older male; for the longest time I was afraid that she would try to "play" with them like she does my cats. But she actually tends to avoid them (because she's been warned so many times) unless I've tossed out leftover treats for them. I trusted my male dog after the first few months, but my female just recently earned that trust, after about 7 months.
I allowed my dogs to see the chicks only over a baby gate at first (I would cup a chick and let them sniff), then in the room with the brooder (with me there of course). Later, when the chicks were moved out to the coop/run, I would sit inside the run with one dog at a time, hand on collar, requiring the dog to lay down, and let the chicks walk all around us. I'd used the command word "gentle!" back when they were still in the house, so that's the reminder I gave throughout my dogs' training. After a week of hand-on-the-collar training (and both did well), I moved on to leash training, still sitting inside the run with the dog, still requiring the dog to lay down while the chicks walked around us, still using "gentle!"
Next came walking around inside the run with the dog(s) on leash, chicks walking about, always using the command word. Finally came taking the dog(s) off the leash, walking around inside the run with them (one at a time always).
When I began letting the chickens out of their run, it was NEVER done unsupervised (at first). I was typically right there, not off doing something else. Anytime the dogs came anywhere near the chickens, I would give them the reminder command, and I would require the dogs to lay down. Later, I would go off a bit, watching from a distance, doing piddly things around the yard. Now, I fully trust both dogs with my girls, whether I'm outside with them or not. I've watched my chickens literally fly across the yard, and my female dog watches, but doesn't move. That was a big concern with her, that she would chase if/when they acted all chickeny and spastic. But she does well. And she is the first one to growl and run at hawks that swoop down in the field behind our property, or geese that land in the lake next door. That tells me she knows that my chickens are "our pack" birds, and not just any old birds.
Consistancy is important. And of course prey drive and what degree of "people pleaser" your dogs have will be a big part as well.
Part of our "off leash" training...