LOL-

K, arguing from the side of my mouth.
Zombies are which
A) Magic- in which case medicine is moot
b) ITS A CONSPIRACY MAN! in which case they are genetically engineered- they are not "DEAD" dead, So are not really corpses.
c) caused by a virus- so KINDA dead, but not really- its just not a Human driving the body- who knows if they can ROT, the virus that drives them might drive out other bacteria/rotting mechanisms.
d) radioactive- in which case, the radioactivity is what kills the rotting bacteria.

At any rate- IF there are zombies, I am less concerned with how long it takes them to rot.
First you gotta avoid the 5 billion shambling corpses.

Then you worry about how long they are going to last.

Then you gotta deal with the loss of INFRASTRUCTURE that was eaten/destroyed/run by former corpses.