How long did Axel hold out before he finally revealed the answer to his uncle? What finally made him to do so?

The next morning Lidenbrock is still at it, fevered and tired. Axel starts to feel sorry for his uncle and wonders if he ought to say something, but the danger of revealing what he has found prevents him. What start to bother him, though, are the hunger pains from not eating. Around noon he starts to feel unwell. Two hours later he is worse and decides to reveal all.