Continuity mistake: Belle's nails disappear and reappear throughout the movie.
Continuity mistake: When Beast is grieving in the rose room before Gaston shows up, the chair has a missing piece of fabric on the left side. When Gaston shoots the arrow, the fabric is now fixed.
Continuity mistake: There are 2 different designs for Gaston's chair that alternate all the time: one covered in dark fur with V-shaped antler armrests, and another with a lighter coloured fur with S-shaped antler armrests. Also, both antlers on each side of the backrest differ slightly.
Continuity mistake: After Lefou is soaked he disappears in the very wide angle of Gaston walking behind Belle. (00:07:33)
Continuity mistake: When Gaston says he'll make Belle his wife a woman with a grey colored hair suddenly appears out of nowhere in the close-up. (00:07:58)
Continuity mistake: When the three girls first see Gaston, the fountain is pouring loads of water into the bucket. A shot later the water has suddenly stopped and the girl in red has the bucket raised up to her waist.
Continuity mistake: When Gaston proposes to Belle he kicks off his boots, which disappear in all further shots.
Continuity mistake: During Be my guest song the menu suddenly disappears from Belle's hands between shots. (00:39:39)
Continuity mistake: When Mrs. Potts puts Chip into the cupboard, the handles change style and colour: from gold to black. Also, the carvings on the wood become blue painted for a brief shot.
Continuity mistake: Beast bursts into the West Wing, uses the magic mirror and puts it on the table. A shot later it has vanished. Note the shape of the rose between both shots and you'll notice that it's not an opposite angle. The mirror is definitely gone.
Continuity mistake: When the Quiet village song ends, the sign on the last house on the right swaps from green to brown coloured between shots.
Continuity mistake: When Gaston stops Belle on her way home, the three blonde girls magically appear in the middle of the village sighing of love, then they disappear but reappear a shot later next to the last house in the village. (00:08:40)
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.