I think that depends on the average calorie deficit.
the old calories in vs calories out forumla
a very very basic way is to
multiply his weight in lbs by 11 = calories needed per day
less calories in per day
then divide that by 3500 to equal pounds lost
then mulitply that by 7 for the week
EG
20 Stones = 280lbs
Calories out = 280*11 = 3080 (Calories needed to maintain 20 Stones)
Calories in = 1200
Calories Deficit = 3080-1200 = 1880
Daily Loss = 1880/3500 = 0.54lbs
Weekly Loss = 3.75lb
Monthly Loss = 15lbs
This is a very very rough guide though, not sure if there is an official CD one - The 1 stone a month SS rule is very average as we know the bigger you are the higher the weekly loss.