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)
Visible crew/equipment: When Julia and Max are talking about Alex Trusk, Max then goes on to tell Julia that he designed the concept for the variable time-speed rooms. As he walks across the room, exclaiming "It was for a game! Now here we are!", watch the floor to the right of the screen, and you can see some black crew equipment laying there, which is nothing that belongs to Julia or Max. (01:08:25)
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)
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 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)
Revealing mistake: When Kate opens one of the doors to find Simon on the other side ("Hello gorgeous, miss me? Huh? Come on in, the water's fine."), the shot of her climbing up to open the door is a recycled shot from 42 seconds earlier. It's pretty obvious as the shot still contains a mistake from when the shot was used previously - a boom mic is reflected in the door's shiny surface as Kate climbs up to open it. (01:18:40)
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)
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)
Visible crew/equipment: After solving the "60659" mystery, Kate opens a portal, revealing nothing but darkness outside it. It then cuts to a shot of the watch ticking, then back to Kate who is still kneeling down. In this shot you can clearly see down her blouse, and the long black mic wire and mic is very visible. (01:22:45)
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.
Revealing mistake: After Kate kills Simon after he kills Sasha, the small hands on the four watches that he is wearing move at different times to each other when they are supposed to be synchronised.
Visible crew/equipment: As all the different realities start collapsing into one space, Kate goes to open one of the doors. As she climbs up to open it a boom mic is reflected in the door's shiny surface.
Other mistake: When Max looks into the room with Julia, we see her lying on the ground from the perspective of the original room. But when Max enters the room, we find out that the room's gravity is different, meaning Julia should really have been lying on the wall, by the perspective of the original room.
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.