there are so many variables that it is really hard to time. I have seen my tracking dog find a hog after 23 hours. then loose a trail after 2. soil, vegetation, moisture, time of day, wind, all play a part, and probably a bunch of things i haven't mentioned. Granted a tracking dog is not a hog dog, but the same basic concept applies.
Are you talking about a blood trailing dog. If so, when they are trailing there, is physically something(blood) there. Trailing a hog, they are just smelling they scent left behind.
