I weighed 334 pounds when I started my weight-loss program, so about 152kg, not quite in the ballpark you are positing, but it might be instructive. I lost 150 pounds in 9 months, which works out to an average of almost 4 pounds a week. The first week was 12 pounds but some of that was water weight. Weight loss was faster at first and then slowed down. We were encouraged to exercise and to increase our level of exercise over time, which certainly contributed to the continuing success of the weight loss.
My weight loss program was a doctor-supervised liquid diet consisting of 500 calories a day (they don’t do that any more, now the minimum is 800) of high-protein shakes supplemented with a liquid potassium supplement and at least 64 ounces of water a day. The doctor supervision consisted of blood tests every month and a short doctor assessment every week (weigh-in, blood pressure, interview). There were also mandatory group meetings every week, intended for nutritional education and support in continuing the program.
So for someone as obese as 250kg (550 pounds) I think 5 pounds a week over a long period of time is reasonable, if they are in a similar sort of program; they could reach 200 pounds in 70 weeks at that rate. I would never advise it without the doctor supervision, even though that was probably 2/3 of the cost (the rest being the cost of the shakes).
**jtur88 **- that’s not the way it works. Your body burns less energy as you lose weight. A 550 pound person at rest will probably burn 4500-5000 calories a day just to feed all the body cells he has. A 200 pound person at rest will only burn around 2000 calories a day (assuming a higher ratio of muscle to fat for a slightly higher burn rate). So if you are eating 2000 calories a day, you will lose weight at a rapid rate when you are 550 pounds, and if you reach 200 pounds you will stop losing. This is sort of a simplistic explanation, nearwildheaven is right that it is more complex than that. But it is a useful rule of thumb.
By the way, in the latter case you can either reduce your calories or increase the calories you burn by exercising, or both, to continue to lose weight. Studies show that in long-term and large-scale weight loss, most of the weight lost is through reducing the calorie intake, and exercise helps; with maintenance of weight after weight loss, exercise becomes more important.