Answer: 0.25 inches Explanation: That is proper procedure. 
RP Enterprises 
Answer: 1 inch Explanation: They need to be 1/4 depth of slab. 
Elite Concepts 
Answer: 1 inch Explanation: I know that control joints do not prevent cracks and its important to put your cuts in the right locations to minimize cracking.

COMMON SENSE FLOORING, INC. 

ProMatcher 
Answer: 1 inch Explanation: No more than 25% of the slab thickness. 
Cronin Construction & Resource Management, LLC 
Answer: 1 inch Explanation: ACI recommends control joint depth equal to 1/4 of the slab thickness. 
Maven Group LLC 
Answer: 1 inch Explanation: Control joints should be 1/4 of the thickness of the concrete slab. 
QUALITY CONTRACTORS 

ProMatcher 
Answer: 1 inch Explanation: I always prefer do it at 1" deep 
alvaradomasonry 
Answer: 1 inch Explanation: Usually one third of the thickness. 
Concrete services 
Answer: Not sure Explanation: Depends on the thickness of the slab. 
BellPro Construction LLC 

ProMatcher 
Answer: 0.25 inches Explanation: .25 inch to a half inch. This can vary. 
Southern Quality Concrete 