A round is 6 seconds, and a commoner has 4 HP. I don't think a commoner wouldn't faint from being covered in stomach acid for 24 seconds.
But of course it's the DM's job to cover this stuff.
My personal view is that individual body parts of a troll don't regenerate (otherwise "loathsome limbs" wouldn't even exist), and that only the main body (the body part including the heart, for instance) can actually heal itself on its own.
BTW I think troll meat would taste horrible. Rubbery and slimy, smelling faintly of ammonia.