The amount of time required for a Route 66 road trip depends on several factors… How true to the original alignment do you wish to be? Do you plan on taking any side-trips? How often do you plan on stopping the car and exploring the towns/sights? How spontaneous/planned do you want your road trip to be?
Driving from Chicago to LA can take only 4 days if you’re prepared to spend all day on the interstates, although this is not the same as driving Route 66. (It is worth noting that if you have a strict itinerary to follow then hopping onto the I40 occasionally is a practical consideration for those with limited time.) Ideally it should take you at least twice as long if you want to get a taste of the real Route 66.
As you get deeper into planning your Route 66 trip you’ll find it increasingly tempting to make the most of the opportunities for great side-trips; the Grand Canyon, Las Vegas, the Santa Fe loop… If this is the case then you should set aside an extra day or 2 for each.
If you intend on getting the most out of the experience then you’ll be stopping frequently along the Route to explore towns, museums and roadside attractions. All of this takes time of course and will limit your day’s mileage.
If you are able to plan ahead a little you may find that there are times when all you are doing is criss-crossing the I40 on the frontage road and you might benefit from hopping on the Interstate if time is tight. This may feel like you’re cheating but there are times when you’re on the interstate that you’ll be less than 50 feet from Route 66! Remember - driving Route 66 shouldn’t be stressful and it’s okay to tailor the journey to your own requirements.
I found that two weeks allowed for a comfortable road trip, including a night at the Grand Canyon and a few nights at Vegas. The days were long but allowed me to experience all that I wanted to cover without ever feeling rushed. However, the length of your trip ultimately comes down to your own personal circumstances. I would recommend no less than a fortnight to cover the full length whilst at the same time pointing out that two weeks makes for a Route 66 trip of a lifetime!