How long will it take to lose 3 stone?

Depends how you're going to do it.

If you exercise, preferably aerobic exercise, like running (I do 1.5 miles 3 times a day... which absolutely kills my legs after the third one...), you should see an initial INCREASE in weight, followed by a gradual decrease. I'm not sure what causes the increase, but the decrease is what we're after, so we'll look at that.

I have been doing steady exercise for about 4 weeks now, and I have gone down from 16.st 10.lb to 15.st 4.lb.

People tell me that eventually, the body will feel rather lethargic (tired), and start to undergo a process known as meiOSis in the fat cells. MeiOSis is a type of division in cells where it lowers the chromosomal amounts by half. According to my Biology textbooks, this would create sex cells, so I suppose that it isn't MEiOSIS that happens, but MITOSIS. Mitosis is a type of division where cells divide, but still have the same number of chromosomes.

Um... I think I've just described you getting MORE fat cells through excercise... I'm pretty sure I've gotten something wrong... Ah well, someone will answer this correctly eventually... :D Good luck with the weight loss!

Just to edit what your saying, if your body starts to get tired, then as you eat food it takes the food and stores it as fat, so even by dieting if you eat too little you will see yourself eventually stop losing weight and even sometimes gain weight, so even when dieting its always good to make 1 day every fortnight and treat yourself so your body doesnt go into this mode.