i think it depends on the ground conditons...hard, soft, wet, dry, etc..
also, i think the weight and size of the hog matters too...bigger boars leave bigger and deeper tracks. Bigger boars have bigger smells..
what do i know though...
my opinion...
hot- 2 hrs or less...
medium- 3-6 hours...
cold- 7-24 hrs...
My dogs do well when they've been there in the last hour!!