@ankara-- I'm not a dentist, so I may be wrong, but I think that the symptoms of a failed root canal show up fairly quickly. This is especially true if the infection is due to the dentist missing a root.
If you still have an infected root in your tooth, it should cause inflammation and pain soon after the root canal.
But it's possible to have a failed root canal for different reasons. For example, if the dentist doesn't seal the permanent crown all the way, bacteria can get in and infect the tooth. That can happen at any time, even a year after the root canal.