I've found in OTB tourney play, at the amateur level, one player goes out of book within 4-6 moves. The longest I've had was 10 or so moves by transposition.
Knowing the moves in a opening is only going to help if you know why the moves are normally played and can understand what to do when your opponent deviates. Add to that, the number of different branches that are possible, the adage to not study openings makes sense, to a certain point.
Understanding principles will go much farther, for most people sub-expert players, than memorizing concrete variations.
I think the common adage of "don't study openings, only learn the rules of the opening" is said far too often.
I think this matters only at the very beginning stages of chess. Say under: 1200-1300. Anything above that I think it's important to pick some openings you're comfortable with and practice them far enough to get a good playable position.
My theory on this is that if you meet a 1300 who knows their openings better than you, you are going to be fighting from a bad position the whole game right from the get-go. You both will blunder, but at least he will enjoy his game more having the better position and he is more likely to succeed.
An example of how knowing your opening makes the difference is this game where a 1700 beats a GM.
https://www.youtube.com/watch?v=91tbGuKVZWk
It's because the GM changed his opening and didn't know the variation he played against the 1700. Knowing to navigate your opening choices and patterns is critical.
In my opinion... you should study your openings.
Study them enough enough to where you will get playable positions. Chessable.com is a great resource for that.
I'm questioning this common phrase and assumption.
"If you find yourself on the side of the majority, it's time to pause and reflect."