People tell me that I should lose no more than 2 lbs a week if I want to keep it off. I was able to do a half a pound a day for my wedding (55 lb total), but I was cold ALL THE TIME and had terrible foot cramps from malnutrition.
The first 6-8 lbs are primarily water, as you said. After that, you have to really earn those losses. I always get really excited during the first period ("wow, at this rate, I'll be done in just X months!"), and then the losses peter out and I really have to knuckle down and find my caloric sweet spot to get going again.