how long did greg hold off the black water of despair?

My opinion would be that he held off the 'black water' of despair until the blindfold went across his face. Greg handled the kidnapping well and even tried to befriend his captors. I don't see him as being in despair until they blindfolded him and put him in the pickup truck. That was the first point he believed he'd be killed......... the despair would have ended when the cloth came off, he saw the feast, and his captors told him they'd help to finance his school.