I'd think most GMs could probably refute bad lines on the spot, and would know every solid response to the opening they play. Most of them have about 5 main openings per color and have a line prepared against all of the main variations. How many moves into the opening depends on the opening. Less common openings they probably only know about 5-10 moves deep, but the most common ones are known 20-35 moves deep.
At GM Level, How Deep of Opening Knowledge Must You Have?
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I currently play two openings for white and two for black for a whopping grand total of four! Yay!
At the sub-1200 level, that's all I need ...if not less...to win. And my knowledge of lines in each opening is only about six to seven moves deep for about three variations of each opening. That is very shallow, but all I need right now.
Anyhow, how deep of a knowledge of openings do GMs need in order to compete successfully at their level?
Do they have to know pretty much all the openings? And how many variations and lines into each variation do they typically know?