Depends on whether you're talking about going from 25 stone to 19 stone or 15 stone to 9 stone. Weight loss will slow down a lot the closer you get to a healthy weight.
I've known several people do it in around 6 months.
What you have to remember is that the odd little set back will have probably no bearing at all on your overall weight loss. Plus that weight loss, when you look at actual loss of fat not fluid is not linear and doesn't happen immediately after a good/bad day etc. At any time your weight can vary by a few pounds for many reason that have nothing to do with the amount of fat you carry.
However you choose to do it, it has to be sustainable and viewed as a lifestyle change, not something you can do for a few months, before thinking 'I'm thin now, I can eat what I like and as much as I want to' because that's likely to put you back in a position where you again need to lose 6 stones.
So you need to think about eating sensible portions of healthy food most of the time, but be flexible enough to cope with the odd takeaway, bar of chocolate, meal out, holiday, etc etc. Because that's what life is.