Answer: 24 inches Explanation: That depends largely on the subgrade materials, and traffic serviced by the driveway, but on stiff clay soils, 24" of aggregate base is typically adequate to bridge irregularities and produce a smooth, friction minimized subgrade. 
Maven Group LLC 
Answer: 02 inches Explanation: Thickness required depending on condition of surface being poured on. 
BAT Concrete llc 
Answer: 24 inches Explanation: Do what's necessary 
Phoenix concrete services inc. 

ProMatcher 
Answer: 46 inches Explanation: The gravel base for a driveway should be a minimum of 4 inches deep. 
Colorado Concrete 
Answer: 02 inches Explanation: 2 inches is recommended 
alvaradomasonry 
Answer: 46 inches Explanation: A good base is as important as the thickness of the concrete because that is what supports the concrete. 
Concrete services 

ProMatcher 
Answer: 46 inches Explanation: base should match concrete thickness 
LPG,LLC 
Answer: 46 inches Explanation: This is as per code requires 
h and h company 