There are many posts in different forums about this, the simple answer is the longer the better. It not logical but in reality you need to do your research on what you want to see and do before deciding on your trip.
Particularly for us coming from Australia, we have noticed a lot of people are tempted to try and do the whole thing in 2 weeks and a side trip to Vegas.
If you do this you will not get the true feeling of Route 66 or Vegas come to that.
Better to split the trips or do part of the Route
Have a look at our Sample Itinerary page, it list some (not all) of the sights and stops on the trip