It typically takes most rookies 2-3 years to become good players. However, I believe you can tell a lot from their success in the Vegas Summer League as a rookie (I don't necessarily give as much credence to a player in their 2nd or 3rd year doing well because they they should do well against the incoming rookies if they are ever going to amount to anything). For example, take a look at the summer league statistics in 2005:
You'll notice a lot of the players on that list are now the most desireable in the league. This is not always the case as some players' play simply doesn't translate or they are stuck in a tough situation for playing time or they simply get injured, but 2005 is particularly compelling as nearly everyone at the top of the list is now a valuable contributor if not an outright star. Now, take a look at this years' statistics:
I would be willing to bet that Bayless, Randolph and Greene (as rookies who did well in the summer leagues) go on to have success at the next level (Tucker and Douby aren't as impressive because this isn't their first go-around).
I don't want to read to much into this, but it is something (as we rebuild) to keep track of.