I wasn't aware Jon was dead for more than 12 hours, if he was gone for days he'd have started to get pretty ripe, especially since magic was not yet involved.
Well it was definitely more than 12 hrs if not 2 days. He was stabbed at night. Davos finds his body (probably early morning) and barricades himself in a room. Thorne gives them until sundown to surrender. Wildlings arrive and take over. More talk and blah bla. Finally melisandre decides to use her magic on him. So if not 2 days he was dead for at least a day and half. He probably didnt rot because of the low temperature, plus the show ignores such minor things.
In the book, I wont be surprised if Melisandre revives him a lot faster than they showed in the show.
Jon looked pretty damn good for a dead guy. I know why he didn't show a single sign of decomposition... it's in the script.