The truth is that you can reach any healthy weight from any size with the right techniques. You can drop from 450 pounds to a size 12 dress for example, without bariatric surgery or even medically supervised diets like Very Low Calorie Diets (VICD). Many have achieved this to confirm that it's a replicable possibility among particular people.
When it comes to total weight lost what is realistic is primarily depended on the individual. The motivation the desire and the willingness to change are very important. So important are these that even extreme interventions like weight loss surgery that have a near guarantee of huge permanent weight loss, weight regain can occur if the individual lacks the desire, will and motivation to change.
However even all the willingness and passion will never replace good dieting habits. Infact when looking at realistic dieting goals you should look at two main perspectives; total amount of weight loss targeted to result to the idea weight or desired weight and the time taken to reach this goal or rate of weight loss.
You have probably heard that you should aim for two pounds of weight loss a week. This is what is generally acceptable as healthy. But as you probably are aware, weight loss tends to taper and hit a platue.
Similarly you may have heard of people that lose much more in a week. For example the story featured on US magazine of Janet Jackson who lost 60 pounds in 4 months. From chubby figures that could no longer fit her rings, she transformed into a model that graced the cover of the magazine with chiseled abs.
What you need to understand is celebrities like Janet Jackson have nearly endless resources to support their endeavor, from motivational coaches to personal chefs. This is why what can be a realistic goal for one is by far not realistic to someone else.
Your goal setting should agree with your level of commitment. Commitment requires sacrifices. It also requires priotisation. Infact there is a level of commitment which if you find yourself in, you should forego losing weight for the time being. This is rather than putting yourself in a position of imminent failure hence frustrating yourself.
And though it seems that you should set goals depending on your level of commitment you can do the reverse. For example, you just want to fit in a size 10 jeans. Then you can find what is needed overtime to achieve this. Then you can commit to achieve it.
All in all what is realistic always depends on what your really want and how much you are willing to pay to get it.