Continuity mistake: In the overhead wideshot right before Maurice walks in there's a green mat by the doorway, and nothing hanging over the large window, but when Maurice walks into the tavern there's a blunderbuss hanging over the large window, and the mat has vanished.
Continuity mistake: After Maurice is thrown out of the tavern Gaston whispers his plan to LeFou, and when they begin to dance we see an angled wall with a dart board, bow and arrows, but the problem is that angled wall with the dart board, bow and arrows did not exist in the overhead shot, right before Maurice walked in.
Continuity mistake: During Gaston's song, when they sing "Give five hurrahs, give twelve hip-hips," LeFou's beer tankard switches from his left hand to his right hand, between shots. Additionally, one of the wall's wood beams near the fireplace has vanished behind LeFou, then reappears. The same thing happens again later, after they toss Maurice out the door.
Continuity mistake: During scenes at the tavern, the shelves between the brew barrels keep changing between being quite ornate to being ordinary shelves.
Continuity mistake: While Beast is bathing in the bathtub Lumière is standing on a stool with two level foot rest bars, but in the following shot the stool has only one level of foot rest bars, then it's back to two levels again.
Continuity mistake: Right before Belle's father enters the tavern, watch the bar on the left: First there's just one man. The father enters and a bartender plus a guy next to him appears. Then Gaston asks "Who's got Belle locked up in a dungeon?", and all three disappear. Then the father says "A beast", and the bartender and first villager appear, but not the second man.
Continuity mistake: After the Gaston song ends there's a wide shot where there's a bare wall by the doors. When Belle's dad enters, a drinking horn hanging on the wall has appeared.
Continuity mistake: While Belle is reading a woman in a house pours water on the drain pipe by an empty window. Belle avoids the water by pushing a wall sign and right then two vases with flowers appear.
Continuity mistake: After the baker cries "Marie, the baguettes!", there's a wide shot of the place and he has disappeared.
Continuity mistake: When Belle sings "Madame Gaston" there's a huge barrel against the wall, which disappears in the further shots.
Continuity mistake: After Gaston walks out of the puddle, a wide shot of Belle's house shows there are no cellar doors, but in the close-up they suddenly appear.
Continuity mistake: Gaston throws the closed satchel with coins at the asylum guy, but when he reaches for it it's open.
Continuity mistake: When Gaston enters the palace he sees a mop. In the aerial angle it has a puddle of water around, which disappears in the close-up.
Continuity mistake: After Belle discovers the rose and Beast shows up, check a green book behind her. When the angle changes a red one has magically appeared.
Continuity mistake: The first time Belle discovers the rose it's facing a 12 o'clock position; when the Beast jumps in, the rose is facing a 3 o'clock position.
Continuity mistake: When the villagers open the door with the tree trunk, they make a hole in the door, which disappears in further shots.
Continuity mistake: When Gaston shoots Beast, the broken part of the window swaps from separated and totally broken, to slightly broken between shots.
Continuity mistake: Right before "Be Our Guest", Lumiere kisses Belle's hand. In the first close shot, his arm flame is lit. The far shot then shows it out. Then the second close shot shows it lit again.
Answer: The Rose started wilting at The Beast's 21st birthday and could have presumably started wilting years before the first time we saw a petal fall off. Given the fact that Lumiere stated that they were living with the curse for 10 years, prince Adam could be 23-24 years old at the time of the original movie,13-14 at the time of the curse, and the Rose could have been wilting for two to three years. This is my best guess given the approximate age of Prince Adam in the ripped up portrait before he was the Beast.