How long does it take to drive Route 66? The flippant answer would be "how long have you got?" but I'm guessing that you're reading this article because you were hoping for a more helpful and pragmatic response!
So here goes...
Assuming that you don't have months to set aside to meander through every historical alignment, soaking up the atmosphere in every cafe/bar in every town, you should try to set aside at least two weeks for a great Route 66 road trip, but preferably three or four if you're hoping for a richer, more leisurely trip. I should point out that this article is aimed at those travelling one-way and not for return journeys.
It's true that the journey from Chicago to LA can be completed in 4 days if you’re prepared to spend all day on the Interstates, but this is NOT driving Route 66. If someone tells you they drove the I40 and thoroughly enjoyed their "Route 66" trip then they've missing the point - Route 66 isn't a case of getting from Chicago to Los Angeles by any means possible. Route 66 is an historical route that follows a complex road network through the many small towns and minor roads that were bypassed by the introduction of the Interstate system.
However, while on the subject of Interstates, it's worth keeping in mind that they're not your enemy - if you're pushed for time and have a strict itinerary to follow then hopping onto the I40 occasionally is a practical consideration if you're pushed for time. If you're on a time-limited schedule and you already know which areas you'd like to spend more time in then don't be afraid to use the Interstate to help you get the best out of your trip. Ideally you won't be pushed for time though as this is sure to compromise your Route 66 experience.
Ultimately the amount of time you need to set aside for a Route 66 road trip is dependent on several factors: Do you plan on taking any side-trips? How often do you plan on stopping the car and exploring the towns/sights? How true to the original alignment do you wish to be? How spontaneous / scheduled do you want your trip to be? Let's look at how some of these factors might influence your trip.
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, Monument Valley… If this is the case then you should set aside an extra day or 2 for each. One night at the Grand Canyon would allow you to admire the views both as the sun goes down and during the spectacular sunrises. I'd recommend more than one night for Las Vegas particularly if you wish to spend one night catching a show. If you wish to follow the Santa Fe loop this needn't be treated as a side trip and will comfortably fit within your regular itinerary.
Route 66 is as much about the stops as it is the driving. 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. Sometimes you might just stop somewhere to stretch your legs or grab a coffee in a local cafe, whereas there will also be times when you will want to see an attraction that will require several hours of your day (for example Acoma Pueblo or the National Cowboy and Western Heritage Museum). Many roadside attractions are simple curiosities that won't impact greatly on your progress (Cadillac Ranch, the Illinois Giants etc...)
If you wish to follow each variant of each alignment then you are certainly not going to be doing this trip in two weeks as this is a very particular way of approaching the Route. Most people don't approach their road trip in this way and follow the generally accepted Route and it's occasional variants as outlined in the EZ6 Guide. This alignment is achievable in a minimum of two weeks whilst still having a great trip.
Also, If you are able to look 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! By following the EZ66 Guide these occasions are made obvious. Remember – driving Route 66 shouldn’t be stressful and it’s okay to tailor the journey to your own requirements. There's no shame in making time occasionally!
Ultimately, the length of your trip comes down to your own personal circumstances. I'd recommend no less than a fortnight to cover the full length - three or four would be preferable - whilst at the same time pointing out that if you only have two weeks you're still going to have a great road trip!
Take it easy, Steve