Good question..I googled it:
"One of the questions surrounding human cadaver dogs is how soon after death they can recognise a corpse, and how long a "fresh" corpse must remain in one place for a dog to detect that it has been there. In a study published last year, the forensic pathologist Lars Oesterhelweg, then at the University of Bern in Switzerland, and colleagues tested the ability of three Hamburg State Police cadaver dogs to pick out – of a line-up of six new carpet squares – the one that had been exposed for no more than 10 minutes to a recently deceased person.
Several squares had been placed beneath a clothed corpse within three hours of death, when some organs and many cells of the human body are still functioning. Over the next month, the dogs did hundreds of trials in which they signalled the contaminated square with 98 per cent accuracy, falling to 94 per cent when the square had been in contact with the corpse for only two minutes. The research concluded that cadaver dogs were an "outstanding tool" for crime-scene investigation."
The CSI death dogs: Sniffing out the truth behind the crime-scene
