Continuity mistake: The position of the 'rope' of clothes on which Simon is holding changes. In one shot he is holding onto a particular part of a shirt, but in the next he's holding onto a different part. (00:31:05)
Continuity mistake: When Sasha is trapped in the Razorsphere room, she yells "Kate, help," and then between three shots her necklace changes from tucked under her purple shirt, to hanging out and back to tucked under. (00:54:20)
Continuity mistake: The blood that Simon wipes off the knife onto his hands disappears instantly. (01:04:30)
Continuity mistake: When Colonel Maguire lies down on the floor he prays and folds his hands. When it cuts to a wideshot you can see that he isn't folding his hands anymore. (00:07:05)
Continuity mistake: When Simon breaks Sasha's neck, she falls to the floor, and lands face down. But during the fight between Kate and Simon Sasha's head is now facing to the side. (01:20:35 - 01:21:40)
Continuity mistake: As everyone leaves the Razorsphere room, Julia leaves her high heels on the floor. Later on in the movie we see as she climbs through a portal, she is holding her heels. In the next shot they vanish. (00:53:20 - 01:02:55)
Continuity mistake: When everyone climbs out of the 'Razorsphere Room' Sasha cowers in the left corner of the side of the room opposite to where everyones climbing out. But you can see in the wide shot when Jerry gets killed by the Razorsphere, Sasha is now in the right corner of that side of the room. (00:53:05 - 00:53:50)
Continuity mistake: When they find the dead scientist, Kate helps Sasha put her left foot on the ladder twice. (00:38:05)
Continuity mistake: After Kate has told Jerry Sasha is blind he says "Oh, bummer, to be blind in this place," then he looks up. When it cuts, he looks up again. (00:12:05)
Continuity mistake: When Jerry is telling about the last thing he remembers was going to sleep we can see Simon's jacket hanging on the handles. When it cuts to a close-up of Jerry the jacket is gone. (00:32:30)
Continuity mistake: When Simon and Jerry are holding Colonel Maguire and Simon says "Look at his hands. Cigarette burns." First Maguire has his hand down, but when it cuts he is holding it up. (00:15:40)
Continuity mistake: When Simon has Mrs. Paley tied to a ladder rung, prior to the slivers of light coming, Simon has two large sweat patches on his grey tee-shirt. Yet when he climbs out of the portal door, away from the slivers of light, the sweat patches have gone in a matter of seconds. (00:59:45 - 01:01:10)
Continuity mistake: Simon ties up Mrs. Paley to the ladder with his belt and threatens to kill her. Then slivers of light start to come out the wall, and he releases her hands from the belt, and chucks it to the floor. Between shots the belt moves and also disappears, only to reappear again. (01:00:20)
Continuity mistake: When Kate figures out the '60659' mystery, in the background you can see Sasha laying on the floor dead. Her hair is covering her mouth, but in a later shot, when Kate takes her necklace, her hair has moved to covering her eyes. (01:21:35 - 01:22:30)
Continuity mistake: Just after Jerry writes the number 4 on the room where he meets Kate and Sasha, they cut to a far shot as he and Kate converse. The spot where he wrote is visible and the marking is gone.
Continuity mistake: When we first see the colonel, he grabs the suitcase to examine it. In the far shot, you can see that the handcuffs are on the far side of the handle, near his body. The next shot is a closeup of the suitcase, and the handcuffs are now on the opposite side of the handle, away from his body.
Continuity mistake: When Kate turns to see all the numbers appear on the silver frame around the portal, although they are meant to be in the same position and handwriting as when they were drawn, much of the writing is not. (01:17:25)
Continuity mistake: When Jerry scratches a '5' with his watch onto the wall, in a wide shot he starts to scratch the 5, then in the closeup of him scratching it onto the wall, much more of the 5 has been completed between shots. (00:17:35)
Chosen answer: I'm inclined to think it was longer than a month because of the lack of proper needed care for such an injury would prolong the healing process. But he only had a handful of souvenirs on him, which means he probably stopped collecting after a while. I concur with your cannibalism theory, there's no other way he would have survived. However that doesn't explain what he could have done for water. This theory of Simon spending possibly months in the cube also brings up the question: If the cube breaks down at 6:06:59, then how is it possible to live in there for months? Well, as we saw, certain rooms have slower and faster rates of time. So, he could have spent all that time in a room with a slow time rate waiting for people to wander into it and feed off of them.