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.
Other mistake: This occurs during the scene where the group has just rescued the colonel. Before Mrs. Paley makes her appearance, the others notice the panel opening, and make movements, but they don't make sense. We'll call the side where the colonel was the north. Sasha is in the southwest corner, and Max is near her. Simon and Jerry are both at the north wall. When the sound of the door opening is heard, Max looks to the west. Simon is kind of in a strange position, but looks correctly to the north, where Mrs. Paley comes from. Simon and Jerry both knew where to look before the door opened, so it's reasonable to assume that it's easy to tell which door is opening, so Max shouldn't have looked at any other door.
Other mistake: At one point, when entering a room, Jerry says that the gravity is changing. This is completely false. The camera moves around, supposedly in a manner to reflect the changing gravity, but if you follow logic and the motion of the camera, the gravity in the room is the exact same of the room that they just left.
Other mistake: When Max looks into another room to see Julia for the first time, he starts to enter the room. He is caught off guard by the gravity shift and almost falls into the room, but is able to grab on the rungs before falling. The way the rooms are built, it would be nearly impossible to do this given that the rungs are recessed around the frame of the door. Max would first of all have had to know that the gravity would shift, and then carefully reach both of his hands around the door in an extremely uncomfortable position to be able to touch the rungs, and would even be able to grasp them until he was actually falling. There is just no way that all of this could conceivably occur as it did. (00:29:00)
Other mistake: In the second scene, where we see the colonel, he is looking through a door to another room. After the door closes, we see that he was looking down, into the room below. But, as we saw in the previous shot, his tie was not dangling downwards as it should have been. It wasn't a Gravity Shift, as he never entered that Cube that the camera was in.
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.