It depends on where I am going and when. Last year, for a eastern trip in August/September, I made reservations well in advance for the Labor day weekend, and for Washington, D.C. for a weekend. That was an eight-week trip. The rest was without reservations.
This summer is different, because I am going up the California/Oregon coastline in Sept/Oct. I already have Labor day reservations at a certain national park in northwestern Arizona, also reservations for San Diego, Monterey, and up the California/Oregon coastline. After I get away from the coast, I will have no advance reservations.
Once on the road, I will often make a reservation one day in advance.