Losing Weight
a lot of factors go in to how saggy your skin is after weight loss. losing slow is certainly much better than losing fast but after years of obesity, most people's skin isn't going to be great regardless. the elasticity and integrity of the skin just isn't there. also most overweight people were involved in yo-yo dieting which makes it even worse since it has been stretched multiple times over the years. it took me more than 2 years to lose my weight and my body was still pretty ravaged - in my opinion. yes, i have seen much worse but still it was a big issue for me. other factors like age, genetics, exercise can also have an impact but they didn't for me. i was 24 when i had my first plastic surgery and i did cardio everyday and strength training 3 times a week throughout my weight loss (and still do). some people say drinking water also helps (again, did nothing for me as i drink water like a horse).
Andy said it all - it depends. My friend who had rny last summer - her skin looks fantastic - except her arms will probably need some ps. Another friend, her throat sags, arms, midsection, legs. It's been five years for her and she needs an overhaul. Both people were about the same overweight for the same amount of years, so it's interesting.
In my readings, give it a couple years after reaching goal to decide whether to do it. Skin can take a while to get back to a semblance of normal. Others may feel that it is inspirational to get it done asap once to goal weight.