Right, it's just a guess. And there's a big difference between top recruit in the country (Okafor or Towns), and lowest 5 star like Mel Trimble of Maryland (who was very good) or Jasean Tate (who didn't have as big of an impact).
In general, these are my expectations for a player at Syracuse:
5 star: Plays a lot, probably starts as a freshman, probably leaves early for the NBA, but if he stays, is at least an All-conference level player by junior year, hopefully sophomore.
4 Star: Might play a little bit freshman year. Gets some run sophomore year and possibly starts. Definitely a starter by Junior year, and is a very good player (hopefully all conference Junior or Senior year).
3 Star or lower: I wonder why we took this kid, but I have faith in the staff. Don't expect anything until junior year. A redshirt during the first year probably makes sense.
Obviously, there are outliers. Waiters and Jerami Grant were 4-stars who were pretty good (or excellent in Waiters' case) their sophomore year and had serious NBA potential (read: athleticism) and left early. Rakeem was a 5 star who was solid but not spectacular until his amazing senior year. Mookie Jones never panned out, and Rautins was amazing. But the above are a good guideline to my expectations for each player coming in.