Our program's basic standard was to shoot for an ounce (28 ml, for those so inclined) every five minutes; if you can do that, great, if not, do what you can do. Some have a fair amount of inflammation in their stomach post op which makes it like dribbling the water through a pinched soda straw, while others have little swelling in there and the water flows through fairly easily. Just sip a little at a time and see how it goes; if it goes easily, try sipping a bit faster, but if it threatens to come back up (or actually does) then slow down and do what you can - it will resolve in time.
I had little problem and was able to go through a bowl of broth (6-8 oz?) and a 4 oz juice box in one sitting (half hour?) in the hospital; my wife on the other hand was much more restricted and it took a while to resolve.