My experience from having done plenty of exquisite corpses like this in the past is that if the instigator doesn't set a limited goal they die pretty quick.
Because they were done in person the goal was easy: Hand the page around the table. 1 panel from each person present.
But since folks drop in and out of the forum I suggest a panel count. Six is cool for a good gag comic to arise IMO, but if folks want to be a little more ambitious, something longer might be doable.