Even though Romero's zombie movies don't exist in 'The Walking Dead' universe, we could maybe look to them as examples of what happens when someone is bitten. In Dawn of the Dead, Roger is bitten in the left forearm and left calf, with a good chunk being taken out of the latter. The leg is subsequently re-injured when a zombie squeezed the bandage while Roger is attempting to hot wire a car. Later in the upstairs hideout, Fran says the infection is spreading fast and she asks if Roger can't be taken to a med unit. Peter responds that he'd seen half-a-dozen guys get bitten and none lasted more than three days. So when Roger finally dies and turns, we knows it's at least three days latter. One wonders if only the arm had been bitten, if Peter had amputated it then and there, would Roger been able to live. In my opinion, it was the bite on the leg that did the most damage. In 'Day of the Dead', Miguel is bitten in the arm while trying to corral a zombie. He panics and goes running through the mine before being knocked unconscious, whereby Sarah amputated his arm with a machete and cauterized the stump with a torch. She says to Rhodes that she believes she got the infection in time, but seeing as Miguel was running through the mine and pumped up on adrenaline, the chances are that the infection pretty much spread through his body at this point; and Rhodes reaffirms what Peter said in the previous movie, that Miguel will be dead in a week or so. We don't know if this is the case, as Miguel commits suicide later in the film only a few hours after being bitten. So, we know from Sarah that the infection can be stopped if amputation is applied immediately to the bitten area; however, three days to a week is how long someone will last if the wound isn't treated.