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.
Revealing mistake: The way Jerry comes into play is problematic. He is shown coming up from the bottom of the room where Kate has just met Sasha. He appears to be climbing upwards, which suggests that the gravity in the room that he is leaving is the same as the one that he is entering. However, the way that he positions his body around the square opening doesn't make sense. He turns around to see the girls. At this point, it would be very difficult for him to stay up, because he no longer has support like he did when he first came up. This shows that the actor was standing on something.
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.
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.
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.
Visible crew/equipment: Just after Mrs. Paley joins the others, the door closes and a boom mic can be seen in the reflection of the door, lowering down a bit.
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.