The body didn't decompose (spoiler)
I don't know how long the body of the young girl lay undiscovered but I would think it was about a week at least.
When it resurfaced in John's bedroom, the body was 'fresh'.
Also, when I did see that girl wrapped up in John's bedroom, I assumed it was the girl we'd just seen on the horse. A sort of revenge murder to make John hand over the money.
Anyone else think that? I know it wasn't her. There were two girls but they did look very much alike.
Also, when John was finally burying the body at the end of the movie, the actress could easily be seen taking deep breaths.