How long do Canada geese sit on eggs?

On site Nigel sees that the titanosaur trail goes downhill towards the volcano, but he must follow it. They find several Mei long which had gone to sleep in a flat-bottomed hollow. Nigel plans to avoid the hollow to avoid waking them, but something seems wrong. He claps a few times, but nothing happens. He pokes one with a stick, but it does not wake. He realizes that the Mei longs are dead from gassing by carbon dioxide of volcanic origin. He looks at his carbon dioxide detector, which gives a reading. He calls out "carbon dioxide!" and orders everybody to go to higher ground.