A Portable Hole (DMG I p.264) is described as being 6' in diameter and 10' deep, which translates to over 282 cubic feet or just over 8000 liters of empty space. But unlike the Bag of Holding (DMG I p.248) which has a weight limit, or the spell Hoard Gullet (Dragon Magic p.68) which has both a weight and volume limit, the Portable Hole is merely limited by its volume.
So how do I know when a Portable Hole is full? Do I have to calculate the volume of each item before putting it in there (and disregarding how to optimally use the space I've got)? Or is there a weight limit somewhere that I missed? Or is this just not mentioned in the rules? I'd rather not haul around a full bathtub for measuring purposes, you see.