I can't put a number to the question. The only deer I have ever lost I searched for over 9 hours. I looked high and low, I would have looked longer, but I had to go home the next afternoon. So, after I shot him, I knew I had a high and back shot, I let him go for over an hour. I had pretty much the same scenario on blood as you mentioned above. I looked all the first day and part of the next, over 9 hours of searching. I, to this day believe that I killed him, but where I hunt if you do not have blood it is almost impossible to track or locate downed game. I believe that the true answer is in the ethics and inthe heart of the person that took the shot. Some people will just say oh well and continue to hunt without really searching, and yet others will spend days looking. My hunting partnet shot a deer several years ago and the only place we found blood was at the point of impact. We searched for 3 days and never found that deer. Even today when we are walking the same area, we are still looking for him. So the real answer is it depends on the hunter him/herself.