You can get a reply from a flier months, even years later. People keep them in drawers, pinned to notice boards etc. You also get replies from roads/areas you haven't ever delivered to because people pass them on to friends and family. That's why they are so good.
Years ago we tried out getting leaflets delivered with the freebie newspapers. It was a total disaster, I don't think they were ever put through any of the doors. For that reason its worth doing a bit of back checking if you can.