I just watched this last night and LOVED it - what an excellent film. I loved the hallucinations and flashbacks, and the whole claustrophobic nature of it. I've recently been to canyons in Utah too, so it was nice to see the landscape again.
I read the book earlier this year and loved it - thought the film was an excellent adaptation. I loved how good James Franco was - I don't think I've seen him in anything else before, and I thought he was excellent.
Honestly, I thought the same thing! Why spend 5 days there before deciding to break your arm?
This guy knew no one was aware of his whereabouts in this untravelled region. He could have snapped his arm after a day or two - enough time to exhaust the other possibilities and overcome pointless hopes for rescue.
There was no need to sit there, risking his life even further by becoming weak and malnourished before amputation.
This was actually addressed in the book. He realised pretty early on that the only actual way he'd get out was to break his arm. But he also realised that he'd bleed to death if he did - the amount of blood released in the accident showed his blood was still flowing strongly. It was only after having been left for several days that the blood flow was cut off enough to slow the bleeding down - when he then stuck the knife in and the blood was so thick and clotted that he realised he'd be able to cut if off without losing so much blood that he'd not make it out. In fact didn't he say that early on in the film, or was that in one of the deleted scenes? There's a bit about 4 options - and he discounts cutting off his arm because of the blood loss. So he has to wait for the arm to start dying before he can cut it off.
Also as others have said - in that situation you have to be really desperate. I'm sure he was hoping for some kind of miracle.