As is said above, it depends on the damage caused, a shotgun or large caliber round is going to be pretty fatal, pretty quickly - three or four minutes of brain function (no consciousness) on the sunny side. Smaller rounds can leave repairable holes.
In theory, if he's got to a trauma team within a (at a rough guess) fifteen minutes and the injury
is successfully repaired, he could continue living the rest of his life normally. HEMS in London have done this sort of operation in the field and successfully saved a number of lives. Normally though, the longer the delay between injury and surgical repair, the greater the degree of disability
due to oxygen starvation
of the brain. Even if they do survive that long they are almost certainly going to be unconscious, intubated and in ITU, which I guess may hurt your storyline.
In order to die after 20 hours, there must be a secondary cause - often multiple organ failure, where the various organs of the body (starting with the kidneys) shut down because they were deprived of oxygen whilst the heart wasn't pumping enough oxygenated blood. Other possibilities inlclude infection leading to septic shock