You need to talk to your doctors office. My surgeon doesn't allow anything (even protein) for the first week post op. just clear sugar free caffeine free liquids. I had a very hard time getting in my liquids the first few days and then I started making myself eat Popsicles all throughout the day, that reallllllly helped!
As for how much you lose and how fast it really depends on your body and how much you have to lose. If someone has 200 lbs to lose they will lose a lot faster then someone like me who has about 60 lbs to lose. I wouldn't worry. Just don't push yourself past the different "phases" too fast. (1. clear liquid, 2. Adding in protein, 3. Adding in some puréed food, 4. Adding in some soft foods, 5. Carefully eating normal.) these each take a certain number of days or weeks depending on your doctor!! Mine does 1. One week, 2. Three weeks, 3. Two weeks, 4. Three weeks. And then if I calculated that all right I get to eat normally NINE weeks post op.
Every doctor is different though, some more cautious then others.