Thintervention is a programme that provides the answer. If you are able to get hold of it, I'd suggest you watch it. It has overweight and obese people struggling to exercise and eat properly. A diverse group of people are given a 13 week challenge. Using a personal trainer and an assistant, the group of 7 people are taught how to exercise (with the inevitable friction and conflict that results when you have a highly fit/motivated person telling unfit people what to do) and eat well, and not over indulge. This is actually harder than it looks, some of the group are real sticks in the mud (such as still drinking alcohol) and other people lie/don't tell the truth about what they are eating. This is brought out in the weekly counseling sessions, when the group leader decides to bring in a polygraph lie detector to ferret out who is telling the truth (nb: the polygraph lie detection is inaccurate, there are more accurate lie detection methods).
It sounds easy, a motivator, and counselling, for 7 weeks. Then the hard part begins. The group are left alone for 6 weeks. They're on their own, like the rest of us. How will they handle it? They all did amazingly well. All lost a large amount of weight. Over the 13 weeks, the smallest loss is about 10kg. Because the range of starting weights was from 76kg to 121kg, you could expect that the amount people lost would vary as well, which it did. But was there a relationship between weight and how much they lost? Yes.
As it turns out, the expected loss can be represented as a square relationship. That means that if you double the persons weight, you can expect that they'd lose 4 times as much weight per week. A simple way to use this to calculate your expected loss per week is to take your weight (in kilograms) and divide that number by 60. That's your expected loss per week as a percentage. Multiply that number by your weight and you've got the "Thintervention" expected loss. As an example, if your weight is 90kg, dividing that by 60 gives 1.5% per week loss, which equals about 1.4kg a week. Note that as your weight drops, so does the percentage loss per week unfortunately. In case anyone checks my calculations and trendlines, I've used the persons middle weight (halfway between minimum and maximum) to calculate the percentage, rather than maximum.
The graph above shows each persons loss over the 13 weeks (the vertical axis is percentage loss each week, the horizontal axis is weight in kilograms), with the diagonal line being the average depending on the weight of the person.
To use this information as an example, if I were to start at 94kg (my maximum when I started my blog) and get down to my goal of 66kg, it would take 28 weeks.