Well, if the monster has the snatch feat, it'd have to do bite damage every round until the character it was eating died, then it could swallow.
If the monster had the Swallow Whole ability, and we give it the ability to do double damage per round biting and chewing a character before swallowing, if they did use their ability and swallowed a character whole before that character was dead, just do the normal damage that swallow whole would confer.
My problem with Swallow Whole as the only option to a monster eating a character is like what Jester said. He ended up taking less damage a round inside the purple worm than he had been outside. There should be the option of the monster forgoing swallowing a character in a given round for dealing lots of biting damage instead. Giving monsters with "Swallow Whole" the double damage chewing ability of "Snatch" fixes that problem.