Continuity mistake: Kate writes the numbers on her left arm, with the pen she got from the doctor. In one shot though, the writing is on her right arm. (00:39:05 - 00:46:45)
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: 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 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: 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 Simon meets up with the "second" Jerry he throws him to the floor making Jerry almost falling through the door. Then Simon grabs Jerry with his left arm, but when it cuts he is holding onto Jerry with his right arm. (01:04:05)
Continuity mistake: The blood that Simon wipes off the knife onto his hands disappears instantly. (01:04:30)
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 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: The properties of the writing on Kate's arm changes throughout the film. The "60659" changes in various scenes. Sometimes the handwriting differs, or the darkness of the writing changes to being fainter, or vice versa. (01:21:15 - 01:21:50)
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: When Kate figures out the '60659' mystery, when she looks at the watch you can briefly see it reading 6:06:15, then in the next shot, the watch is now reading 6:06:02. (01:21:40)
Continuity mistake: At the end, when Kate is standing on the edge of the exit of the Hypercube, you can see her through the exit in the shots from below. In the shots from the top however, the black door of the exit is still closed. (01:22:35)
Continuity mistake: At the end of the movie when Kate is killed from behind, the number and positioning of soldiers in front of her is completely different in the shots immediately before and after Kate's death. (01:29:00)
Continuity mistake: Throughout the movie the blade on Simon's knife changes. In some shots it's a double straight edge blade, in others it's serrated on one side.
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: 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.
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.