Revealing mistake: When Becky meets Simon in the cube, they hug, then Simon literally stabs Becky in the back. As she falls down to the floor, her black shirt reveals not only some of her chest, but part of a black microphone wire. (01:15:15)
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)
Audio problem: When Simon opens the portal to discover Jerry walking upside down, Jerry is saying things like "Do you need a hand there? You're upside down", yet his mouth doesn't move at all. (01:10:25)
Visible crew/equipment: When Sasha says "I wish I had your spirit" to Kate when Kate is climbing up to open a door, you can see the reflection of the boom mike moving as Kate is climbing. (01:19:05)
Visible crew/equipment: When we first see Mrs. Paley coming through a door, reflected on the shiny surface of the door is a boom mic. (00:16:35)
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)
Visible crew/equipment: When we see first Kate open a door, it opens, and she leaves her hand up, then slowly lowers it. Look to the left of the cube-shaped doorway, and you can see in the room behind her, the edge of a stand or table or some equipment, which obviously shouldn't be seen there. (00:07:25)
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)
Visible crew/equipment: When Mrs. Paley opens one of the portals, she sees herself getting killed by Simon, then Simon getting his head taken off by something. Then the portal door closes, and a camera and crewman are reflected in the portals shiny surface. (00:42:30)
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)
Revealing mistake: When Kate decides to ditch Maguire and escape herself, we get a shot from above showing her face and Maguire's feet. However we only see his trousers. His feet are gone. Even though some could say that the wall removed them, but we can also see that the wall hasn't reached the part of the feet that are missing. It appears the CGI team forgot to add his feet in. (00:21:35)
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)
Visible crew/equipment: When Maguire raises his hands together to say the prayer, you can see the black wire of a microphone, running up into his shirt. (00:06:50)
Revealing mistake: When Colonel Maguire is hitting the walls with his suitcase, there is a shot where the suitcase is coming towards the camera. Dust and stains reveal the protective glass that is in front of the camera. (00:06:10)
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)
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.