Although there is really no good evidence for it, we traditionally describe a "normal" postoperative ileus as lasting 2-3 days, and "paralytic" postop ileus as >72hrs. The small bowel will normally return to normal function before the colon, which is why we hear bowel sounds before the patient starts passing gas or pooping.
The duration of the ileus will be dependent on a number of factors, including the type of surgery, the type of anesthesia, any postoperative narcotics being used, and whether or not the patient is being fed.
As mentioned above, walking is a commonly accepted method to speed the return of bowel function, and some studies seem to bear it out. Interestingly, gum chewing has also been shown to reduce the time to ileus resolution, as has early feeding, NSAIDs, and laxatives.
The General Surgeons love asking about flatus. Personally, I don't. Too often patients either don't know they are passing gas or don't like to admit it. There is also some question as to whether flatus is a true marker of ileus resolution.
I try to get them out of bed as soon as possible, keep them on a good bowel regimen, and wait for stool. Ortho doesn't see nearly as many paralytic ileuses as General Surgery, so that usually does the trick.