Sam, the physiology of the human kidneys and urinary bladder are such that the methods Dave uses are relative safe for the models and reliably produce the desired result.
Normal human kidneys are capable of producing about one liter of urine per hour when functioning at their maximum capacity. For them to function at maximum capacity, there must be a need for the kidneys to do so, or, in other words, there must be sufficient water in the body in excess of its normal requirement for the kidneys to remove it at that maximum rate. However, radically overloading the body with excess water (more than about one to one and one half liters per hour) can be dangerous, even deadly, depending on how rapidly it is ingested. So, for a model NOT to respond to the 40 ounces (about 1.32 liters) in about one to 1.5 hours would mean she was either seriously dehydrated to begin with, was exercising and perspiring profusely during that time period, or has abnormally poor kidney function. Also, holding much longer without urination can cause a model to have obstructive hydronephrosis, a painful but temporary condition in this case that is easily reversible with urination.
As you know from all the bladder calibrations that have been done (holding contests with measured results), the bladder capacities of the various models are not uniform. For each it depends more on the volume within her pelvic girdle into which her bladder can expand and the strength of the muscles controlling the bladder sphincters than on the volume of water consumed.
There --- undoubtedly more than you ever cared to know.