To some extent, that rather depends on what you weigh now.
If you're talking about dropping from 13 stone to 10 stone then that'll take a bit longer than it would to drop from 15 stone to 12 stone, if you see what I mean. As your weight decreases, so does your Basal Metabolic Rate - so weekly losses aren't as big as you get nearer to goal. If you're several stones away from having a BMI of 25, you'll lose at a stone a month (maybe more). With only a couple of stones to lose, it'll be more like ten pounds a month (after the first month), I'd say.
Edited to add: Of course, the younger you are, the faster you'll lose because your Basal Metabolic Rate is naturally higher anyway.
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