Audio problem: When Kate ventures into another cube room, searching for Max because he appears to be moving from door to door very quickly, she climbs through the door and says "This guy sure moves quickly, he's just..." in a wide shot, but her mouth doesn't move at all. Then in the next closeup of her, she manages to move her mouth to the word 'gone.' (00:09:50)
Audio problem: When Jerry opens one of the doors, he sees Max holding up the colonel who hung himself. Max shouts "Help Me!" And you can clearly see his mouth never moves when this is said. (00:13:20)
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)
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.