Your opening preparation ends at a specific move in every game. Knowing where that move is tells you what to study next, and usually it is not more theory.
Ask a club player how deep their opening knowledge goes and you will hear a number of moves. Ask their game history the same question and the answer is usually smaller, and far more useful, because it comes with the positions where the knowledge ran out.
A book move is one that appears in a reference of known opening play. It is not a judgment about quality. Calling a move book says only that this position and this reply have been played and catalogued often enough to count as established theory. Most review tools mark those moves separately and leave them out of accuracy, because getting a memorised move right says nothing about how you play chess.
Two things follow. A long book line does not mean you played well; it means you played what everyone plays. And the moment the game leaves the book is not a failure either. Every game leaves it eventually, and that is where your own chess begins.
At club level the answer is usually shorter than people expect. Main lines with a heavy following can hold for a dozen moves, while sidelines and offbeat replies drop out within four or five. Faster time controls leave the book earlier still, because both players deviate on instinct rather than memory.
The depth also depends on who deviates. If your opponent leaves theory on move five with a move nobody plays, you are out of the book at move five too, holding preparation you will never get to use. This is the most common way club preparation goes to waste, and no amount of extra memorisation fixes it.
If you review one moment from your openings, make it the first non-book move in the game. That position is where you stopped recalling and started thinking, and it is the fairest test of whether you understand the opening or just remember it.
Ask three questions about it:
Opening names are less reliable than they look. The same position can be reached through several move orders and get labelled differently in each game, so a report that groups by name can scatter one structure across several rows. If a line looks unfamiliar in your statistics, check the position rather than the name.
The reverse happens too. A single name can cover positions that play nothing alike once one side chooses a different setup on move six. When you judge your results in an opening, judge the structure you actually reach.
Most players respond to opening problems by learning more moves. That helps only when you are losing inside the book, which is rare below master level. Far more often the game is fine at the end of theory and goes wrong in the next ten moves. In that case the useful study is the middlegame that your opening produces:
Learning three moves deeper wins you one move of comfort. Learning the plan wins you the next twenty.
Sometimes the opening really is the problem. The signs are specific: you score poorly with it across a large number of games, the losses come early rather than in the endgame, and the positions it produces are ones you dislike playing. Sharp gambits pay off for players who calculate well and punish everyone else. Slow systems suit players who like manoeuvring and frustrate those who want a fight.
Before switching, check the sample size. Twenty games is a hint and not a verdict, and a bad run inside a good opening looks exactly like a bad opening for a while.
Statmate marks book moves against a reference of more than twenty thousand positions, so every analysed game shows exactly where theory ended. The Opening Lab groups your history by opening with your score, your accuracy and how deep you stayed in the book, so the openings where you leave theory earliest and score worst are easy to spot.
Because book moves are excluded from accuracy, those percentages describe the chess you actually played rather than the moves you memorised.
Book depth is a fact about your games, not a measure of skill. Find the first move where each game left theory, look at what you did next, and spend your study time on the plans and structures that follow rather than on three more memorised moves.