I too have lost alot of weight 120lbs to be exact, and i can tell you from my own personal experience that i have a little bit of lose skin under my stomach, which used to be way worse before i started working out. Here is what i know about lose skin: Age is a factor the younger you are the less time it will take for the skin to shrink. Another factor is how much water you drink. I read in an article that lose skin is essentially dead skin which in turn makes it dry, but by drinking alot you help bring it back to life so to say and it helps it shrink or tighten. Patience is also very important according to this article it can take up to 2 years after your weight loss for it to go back to normal, well more or less normal. And again i have a visible six pack and fairly defined arms. As for the little bit of extra skin under my stomach i still have a year to go according to this article until i can expect to see it gone.
Another thing worth noting is that many contestants from The Biggest Loser who have lost way more weight then i did have very defined bodies come finale time. I have also read that it depends on how quickly the weight was lost. I lost 120 lbs in 11 months which is insanely fast (wouldnt recommend that for anybody btw) and that is the reason why i had lose skin in the first place. Many fitness coaches would actually recommend to lose only about a pound a week, and at that rate you do not have to worry about lose skin at all as you are giving you body enough time to tighten up the skin. Especially if you are losing this weight with a good workout program and only minimal calorie deficit something like only 500 calories a day.