Also depends on humidity, rain and other weather factors.
Then there's the level of training and the capability of each dog and the handler. Some dogs alert in very subtile ways that are easy for the handler to miss, especially at the end of a long hard day.
I've helped out with our local K9 Rescue group, I have a friend that owns a Black Lab that is Live Scent certified in PA and MD. The dog is currently working on Cadaver certification. During a training the dog located a small cemetery that they knew was in the area with the latest grave from 1721.
The Rescue group has a human foot they keep in an ammo box that they use for training. It is kept refrigerated except during training. I have seen their Cadaver dog follow that scent over a half of mile through the woods, into building, into the bathroom of that building then into a bathroom stall to local the remains behind a toilet. It is truly amazing to watch these dogs work.