Have to agree with Becky on that one. So many factors will control how much you lose.
I'm only 5 weeks post op but including the 1 week pre-op diet I have lost 39lbs (which is 3lbs short of 3 stone). I'm now losing at the rate of 4-5 lbs a week. But I had a very high start BMI of 49 and was 1lb short of 24 stone when I started my pre-op diet one week before surgery.
The younger you are, the higher your BMI, the taller you are, all seem to be things that speed up weight lose .... anecdotally at least. Also, guys always lose much faster.
But I am sure genes play a big role it. I know that anytime i actually stuck toa diet previously I always lost weight consistently and quickly - I am guessing thats just how my body works.
As a rough guide - from doing plenty of research on US forums - anyone starting out over 300lbs should likely lose about 7 stone in the first 6 months. The further under 300lbs you are the less you generally lose.
Hoepfully thouh a few more long termers will post what their average was because I'd be very interested to know! i don't think I'll be lucky enough to stick at 4-5lbs a week forever lol