Answer: It’s better to think of the goal in pounds of fat loss and let the percentages fall where they may. We measure body fat as a percentage of total body weight but calculate the loss in pounds. This is because people who lose fat, especially in health clubs (exercising, taking supplements, etc.) often gain muscle during the same time frame, so total body weight does not accurately reflect one's progress.
Example: If you weigh 200 LBS and have 40 LBS of body fat, then 20% of your total body weight is fat. If you project a pound per week loss of body fat (and you gain no muscle), you will weigh 188 after 12 weeks, and you’ll have 12 fewer pounds of fat, leaving 28 LBS of body fat. The final statistics are now 28 divided by 188 = about 15% body fat.
The lower your body fat, the slower you should go. One pound of body fat per week once you get closer to your goal is about as fast as one can go without much suffering (e.g. hunger and time spent in the gym). If your body fat is moderately high (males greater than 20% and females greater than 28%) you may lose at a faster rate, typically 1.5-2 pounds/week. For those who are significantly overweight and very high in body fat, three pounds weekly may be realistic. However, don't be fooled by the rapid weight loss that occurs initially and expect this rate of loss every week. For someone with significant weight to lose, the alteration of eating and exercise habits can lead to a large loss of body water, showing a 5-10 pound drop in a couple of weeks. Once the fluid losses stabilize, the previous recommendations apply.
Just remember, as you get closer to your individually realistic goal, slow the loss down in order to adjust to your new lifestyle and to help you maintain your new physique. If the process you use to lose is too intense and uncomfortable, the likelihood that you will be able to maintain a lifestyle that enables you to keep the new body composition is lower.