The story is told fifty years after the event. It ends with these words:
I plastered it up. Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. In pace requiescat!
If your question is something like, "How long after deciding to kill Fortunato did Montresor actually succeed in doing it?," the exact timing is not specified. Poe has Montresor state that he intends to take his own sweet time about exacting his revenge. Poe's reason for having Montresor make this statement is that it will give Montresor plenty of time to show his friendship to Fortunato and to declare his friendship to everyone on every possible occasion. When Fortunato disappears, no one could possibly suspect his good friend of foul play! Montresor will also have time to refine his plan and wait for the ideal opportunity to put it into effect.
At length I would be avenged; this was a point definitely settled--but the very definiteness with which it was resolved precluded the idea of risk.
We can only guess at how long it took before Montresor was able to put his plan into effect. He needed to find Fortunato alone in the street. He didn't want anyone to hear him asking Fortunato to sample his Amontillado. Montresor had to devise a story that would make Fortunato want to sample the wine immediately. If there was any delay, Fortunato might tell someone he was going to Montresor's palazzo. Montresor is anxious to avoid the slightest suspicion. That is why he tells Fortunato he is on his way to Luchesi's. This is a lie. Fortunato believes that if he doesn't accompany Montresor to his palazzo immediately, Montresor will go to Luchesi, and Fortunato does not want Luchesi to hear a word about a cargo of Amontillado newly arrived from Spain and being sold at bargain prices. It is the bargain that interests Fortunato. He could make a lot of money if there is a whole shipload of the Amontillado available. He doesn't need to sample Montresor's (nonexistent) Amontillado. He could go to the harbor and find the ship easily (if such a ship existed). He could sample their wine on board and deal directly with the captain or purser, or whoever is handling the sales. But he can't let Montresor go to Luchesi. Therefore, he insists on going to Montresor's palazzo immediately in order to give his expert opinion on the quality of the wine.
“Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.”
Thus speaking, Fortunato possessed himself of my arm; and putting on a mask of black silk and drawing a roquelaire closely about my person, I suffered him to hurry me to my palazzo.
This could have occurred a year, or two years, or three years, after Montresor had made his decision to kill Fortunato. Montresor was in no particular hurry. He wanted to make sure of committing the perfect crime.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.