NEVER actually.
I think sometimes there is an industrialized affinity/compulsiveness for the personal weight scale (rather than a yearly or seasonal health clinic scale) that can create a lot of mental distress with frequent weighting, or once an obese person is within range of a healthy “weight” and the scale stops changing and then life is considered a miserable “plateau” rather than a deeply changing, deeply embodied ongoing process.
Dr. Phinney & Dr. Volek amplify the irrelevance of weight compared to physiological healing and body recomposition several places in their stellar book The Art and Science of Low Carbohydrate Living: An Expert Guide to Making the Life-Saving Benefits of Carbohydrate Restriction Sustainable and Enjoyable. I’m re-reading it for the third time and keep learning new things (plus the quick recipes at the end are phenomenal). They well explain that it’s not about weight per se it’s about body recomposition - which means different things for different people. With LCHF/keto, the body will prioritize the repair of atrophied muscle as it transforms fat. As such, body recomposition is MUCH more a matter of measurements rather than some vague notion of “weight”.
And, I started LCHF/keto as a non-obese (though big boned and skeletally speaking never petite) person - a different kettle of fish as they say. I know weighing for obese folks is encouraging when there’s 100+ pounds of excess fat, and the scale changes are dramatic feedback! But in my case, being not officially overweight by much if anything, I’m more concerned with health benefits and losing the “midlife middle” new belly fat that appeared after I turned… 50.
And since I went LCHF/keto last summer I’ve only measured - with fantastic results (such as 4 inches lost from upper waist, and really loose clothes etc) that encourage me onwards in my self-experimenting adventure.
Being that I grew up with a mother and sister who obsessively “dieted” and weighed their food and their bodies, and primped & preened yet were emotionally/mentally not very healthy or happy - my personal quest (inspired by a dear, tough, beautiful rural-farming raised grandmother who couldn’t care less about complying with beauty standards) became to love my body, inhabit my body, and aspire towards health & healing.
Our bodily expression of watery changes are connected to the ebbs and tides of the Moon’s gravitational effects on the ocean, as well as to how much water we’re holding in our digestive tract and in our changing fat cells (which replace fat with water for some weeks or months before releasing the water and shrinking). We are watery mammals of an ancient earth and its oceans. It is necessary and normal to fluctuate weight daily depending on food eaten, salt levels, and phase of the moon (for real), and this is worth remembering.
Biology is magnificent, and it comes with considerable variation and fluctuation in body water percentage based on a number of factors like age, health, weight, whether one is female or male, the strength of one’s digestive tract/enzymes (hail, Ginger supplements), and what one ate 24 hours ago… 
In fact - for some - the weight on a scale may NEVER CHANGE - but 2-6 clothing sizes are dropped, radiant skin increases, muscle definition enhances, brainz work great, and all the other stuff mentioned in some posts about keto healing in this forum. 
exploded 