It depends how much you weight now and how fit you are - and whether you plan to eat cakes and crisps after you exercise! A stone is a lot to lose in one month and as well as making you feel dizzy and miserable, is likely to go straight back on. Aim to lose a couple of piunds a week for a sustainable weight reduction. As a general rule, moderate exercise 3 times a week, together with a sensible eating plan (eg cut out junk food and eat more fruit and veg) and you should see a difference in a couple of weeks.