There's a video for the incentive spirometer from OCC. It says that it should be about 2500 to 3000 ml.
Here's the link: https://www.youtube.com/watch?v=bt2701U34d8
hope that helps.
It's only my 3rd day of liquids, and I'm feeling pretty hungry. But it's manageable! I've been going to bed earlier, because that's when I am the most hungry! I'm down over ten pounds and very close to my desired surgery weight.