it's going to vary pretty widely because it depends on so many factors besides starting BMI (although that's a big one, of course). Gender, age, metabolic rate, activity rate, what percentage of your body is muscle - those all play a role.
that said, I lost 57 lbs before I had surgery (between my six-month supervised diet that was required by my insurance company, plus the two-week liquid diet required by the surgeon). At one year out, I was 129 lbs down from my surgery weight, and 186 lbs down from my all-time highest weight. I went on to lose another 50 or so lbs during year 2 (have gained 20 of it back since then, which is pretty common. A lot of us experience a 10-20 lb rebound gain after hitting our lowest weight).
P.S. Just noticed this was posted in the gastric sleeve forum - I had bypass. Average loss tends to be a little more with bypass (although not by much), but it totally depends on the person. If you really stick to your program, you can be very successful with either.