So, I know you reverse-pickpocket them into the pockets of the people you want to kill, but when do they eat them? I waited even over 24 hours (In Game) for a target to eat the apple. I checked their inventory and they still had the apple, but they didn't eat it. Also, they had no other food in their inventory as I removed it before placing the poisoned apple.
Most NPC's have a scripted eating time sometime during the day. Certain ones may not however, like most randomly generated enemy NPCs like bandits. Also, they will randomly eat something from their inventory or nearby, so you may want to make sure all other nearby food "disappears" to ensure when they do eat they'll eat the apple.
"How ridiculous and how strange to be surprised at anything which happens in life!"