Once the remains decompose, I've heard you are
supposed to coat the remains with something....
you can get it at the craft store....I want to
say neoprene or....I can't remember but If I do,
I'll post more. Its something to preserve the shell
as the scutes (colored part of the shell) will eventually
fall off and sometimes the shell will fall apart.
I found out about this because we were at an art
show last year and someone had made craft items with
red eared slider shells and I asked her how she preserved
the shells to keep the the natural color and she
said she coated them with....polyprolene? Something.
I was somewhat concerned as to how she had so many RES
shells, but decided I didn't want to go there.
Anyway, it shouldn't take a year, more like a summer at most.
I lost a turtle and dug him up 2 or 3 years later and I could
not find any sign of his head, the scutes were coming off and
the shell fell apart at some of the seams, so you will need to
keep an eye on the remains and pull them as soon as they are clean.