It is a very popular RV trip and I suspect many thousands have probably done it either all the way or along significant portions thereof.
As a straight through trail route 66 hasn't existed for probably 30 years and what remains is bits and pieces here and there. And even back in its days there were many realignments as they continually improved the route. So anybody truly into following Old 66 could probably spend a whole summer doing it. As a practical matter though I suspect most of us just sample as much of it as we can find the time for.
A vast body of road guides and history exist for Old 66 so be prepared to enjoy many hours of reading before you ever even think of heading on out.
Good traveling.