If it takes someone 15 months to lose 15 lbs (rather than, say, losing it all quickly in 3 months as they had hoped) then they've arrived at the same endpoint of being 15 pounds lighter, but they've had an extra year to learn the habits of healthy lifestyle maintenance.
And if it takes 15 month to lose 15 lbs, and not three months, this means that 15 months after the start of your weight-losing journey, you're under the weight where you started... whereas someone who lost them in three months has gained some again...
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What worked very good for me was a 'magical number'...
My weight has gone up and down for a few decades between 50 and 58 kg (No, I'm not skinny, I'm just short.)
Only a short time after having reached my goal-weight of 50 kg, I was going up again, never stayed on my weight for more than a few months, or even weeks.
Until I decided to go down to 49 kg... Once I was under that magical number of 50, my weight hasn't gone up above that weight anymore. The difference between 49.5 and 50 sounds so much more than between 50 and 50.5, that I was much more motivated to keep the 500 grams off that kept me from reaching 50 kg again, than to keep the 500 grams off that seperates 50 from 50.5.