When you go low or zerocarb, you'll lose 5-10lbs in the first days/week, because for each gram of carb your body stores as glycogen it needs 4grams of water, and it has around 200-500grams of it. So when you stop feeding your body carbohydrates and burn it all, you will lose water-weight fast.

To lose 50lbs it took me around 4-6 months, but I definitely wasn't in a hurry and after the first weeks I was already enjoying a life so much healthier that it wasn't a struggle to continue. Now there are 2 years I'm on it and never felt better. I don't need to lose weight anymore but a lowcarb diet is also great to lose more unneeded bodyfat and gain lean mass.