Hi Jessica. My answer would be “never” or at least “almost never.” In my opinion, part of the dysfunction that we (society) have with weight/body image/fat/diet is that we rely too much on the scale! I have worked with many overweight or obese clients (one losing about 135 pounds), and almost to the person, they come to me with a goal of “losing weight” whereas the real issue is losing bodyFAT. Bodyfat is the true culprit, as it’s more closely related to health and health issues. If you help your client focus on reducing bodyfat, the pounds will also come, and perhaps more importantly, her health will improve.
How to measure? Keep her focused on how her clothes are fitting as you make inroads.
My two cents!