Their body purges it out fairly fast. One study I looked at was that the half-life of cyanide in humans was roughly 20 to 60 minutes. That's humans not chickens and I got that over the internet so I can't say it is real reliable, but I'm not too worried about it.
It's not just apple seeds. Practically any fruit seed has cyanide in it. I have many different fruit trees in my orchard. I have absolutely no problem with the chickens foraging in there. I found a plum pit in the gizzard of a chicken I processed, so they will eat them.
The way I look at it with fruit seeds is that a few won't hurt them. But if I make jam, jelly, or something where I have a huge pile of seeds, I dispose of them in a way the chickens can't get to them. If I dumped a bucketfull of seeds in front of them, would they actually eat enough to hurt themselves? Probably not but I don't know that for sure so I take what I consider reasonable precautions.
But if I use a few to make a pie, that goes on the compost heap where they can get to them. There are not enough there to hurt them.
