I'd be careful of losing that much really quickly, and 12m is quick if that's your aim. It will lead to an awful lot of loose skin - one of my friends did that with a gastric band and he is still waiting for an operation to remove skin 3 years on.
I'd aim for 3st/ year. That's around 1lb a week if my maths isn't letting me down, and very realistsic to achieve without setting up bad habits and without stressing your body too much.
So 3 years ish, though of course the first stone will be quicker than the second etc.