I think it depends on what kind of hit it was. If all signs point to a bad, non-lethal hit (i.e. front leg muscle hit, very low in the brisket), then I wouldn't spend days looking for that deer. Most of the time, deer are going to survive those marginal hits, so if you lose the blood trail, comb the area and come up with nothing, it's probably still alive and long gone.

However, if you are pretty sure you made a more solid hit (one lung or a little far back, etc.) then I would stick with it until you find the deer or are unable to keep searching. One example- last year my buddy hit a deer with what he thought was a perfect hit- turns out that it was probably a 1 lung shot due to the steep angle. He made the mistake of searching too quickly instead of letting it bed down and die. Long story short, we tracked the deer for over 400 yards through a swamp into the night. We finally lost the blood trail and couldn't figure out which way he went. My buddy decided to give up and go home. Another buddy found the carcass of the deer (we knew it was the same one because of the size & shape of the rack) several weeks later during gun season- he found it less than 100 yards from where we had stopped searching. If my buddy had come back in the daylight the next morning, there would have been a very good chance of finding it. Moral of the story- keep looking!