We project the dates upon which progressively smaller pant sizes will fit after a step-change in eating habits.
From Projecting Weight Loss we get a formula for future weight. Add to that current hight and waist we can convert this to a formula future waistline.
Solving for standard pant waists, 44, 42, 40, 38, etc, we can predict when new or favorite clothes will fit.
Notes
Waist to height ratio (WHtR) is a measure of body fat distribution. wikipedia 
Body mass index (BMI) is a heuristic proxy for estimating human body fat based on weight and height. wikipedia
Exponential fit to the Minnesota data shows that half time for change in waist girth in lean male subjects was 3.25 wks compared to 8.8 wks for total mass.