They say it takes a stone to drop a dress size. For me that's roughly accurate, I'm an 18 at 13.7 stone and at the top end of a 16 at 12.7 though I think I'd need to be 12st to be a totally comfy 16.
So assuming it's a stone and assuming we're not counting the first week's big loss...
On average, if you stick to plan, the loss should even out to 2lb a week (sometimes 1, sometimes 3 but over a month it usually evens out.)
Which is roughly 8lb a month. So to lose a stone, without pressuring yourself - 7 weeks.
Everyones different so for some people a little more, for some less.