Continuity mistake: When Maguire grabs his suitcase and is about to open it, we can see his left foot stretched out on the floor. When it cuts to a close-up of him opening the suitcase we can see his foot is crouched. (00:06:00)
Continuity mistake: When they are lowering Colonel Maguire down after his suicide attempt, Max is looking down to his left. When it cuts he is looking down to his right. (00:14:20)
Continuity mistake: When Simon jumps into the room where Maguire is attempting his suicide it cuts to a shot from above and we see Maguire's foot is hanging over Max's foot. When it cuts Maguire's foot is out of sight. (00:14:00)
Continuity mistake: When they are lowering Maguire, Jerry is holding onto Maguire's jacket behind his shoulder. When it cuts Jerry's hand is in front of Maguire's shoulder. (00:14:15)
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: When they start running from the moving wall Jerry grab Sasha holding her left hand. When it cuts he is holding her right hand. (00:20:05)
Continuity mistake: When Jerry marks the door with "5" he place his palm flat on the wall. When it cuts, his fingers are bent. (00:17:40)
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 opens the door to the room Sasha is in, we see her from behind pushing her hand to the door. When it cuts as the door open she suddenly has her hands placed on both sides of the door opening. She could not have moved her hands that fast. (00:10:40)
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: In one scene Jerry draws a cube with his watch. There's then another shot, then in the next shot of Jerry, the cube drawn is totally different from what he drew a minute ago. (00:26:15)
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: When Kate is climbing up the ladder to see where Max keeps disappearing to, the left half of her outer tee-shirt alternates from tucked into her trousers, to hanging out. (00:09:55)
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: When Simon and Kate get separated the first time Kate opens the door and lowers her hand. When it cuts she still has her hand up. (00:10:00)
Continuity mistake: After everyone releases the colonel from his belt and lay him on the floor, Simon takes out his identification. Between a wide shot to a closeup, the top of the colonel's shirt goes from being tucked under his tie, to being untucked from under it. (00:14:30)
Continuity mistake: During the movie, Max's shoes change, and they obviously can't because he's trapped in the hypercube. When helping the colonel who's hanging himself, you can see Max's shoes are black, but then later on they're blue. This is most noticeable in the scene when Max and Julia are talking, and Max throws his shoe across the room. (00:13:45 - 01:08:10)
Continuity mistake: When Simon first takes Kate down, he covers her mouth with his hand, then with the other hand, holds a knife. In one shot he holds the knife to the side of her face, but in the next closeup shot, he has the knife resting on his other hand, which is covering her mouth. Then in the next shot he is holding the knife to the side of her face again. (00:08:10)
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.
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.