Opinions vary greatly on whether opening study should be emphasized at all for players below the professional level, so getting into a philosophical debate over the question is probably not going to be of great use, nor would it resolve anything. Rather, I'd like to offer some practical observations and guidelines that have helped me in making effective repertoire and study choices. Regardless of the emphasis you may choose to place on opening study, everybody has to have something prepared for the opening phase of the game.
The primary objective for me out of the opening is to get a playable middlegame position, not to try to achieve a winning advantage. This is not the most aggressive approach, but it's not a bad one in terms of results, either. The goals of all my opening lines are therefore to: 1) avoid losing to forced tactics in the opening, and 2) reach a middlegame where I evaluate my chances are no worse than my opponent's.
Goal #1 (not to lose!) is of course the most important limiting factor. Regardless of your opening choices, there will be at least a few (sometimes many) lines where the opponent can sacrifice material or undertake a provocative maneuver that gives them an early attack. For example, if there is a tactical sideline with forcing threats that eventually ends up equal after 12 moves into the game, but you only know through move 7, your preparation is both incomplete and risky. At the Class level, opponents are typically not "booked up" and will play interesting-looking or aggressive moves in the opening rather than "knowing" they ultimately do not work or end up in equality. This forces us to be prepared not just for lines that theory considers best and popular, but for threatening sidelines. Examples:
- Slav Defense, Geller Gambit. This pawn sacrifice normally gives White an initiative until around move 16-17 and Black can easily misstep if he does not understand the line. You can see my simul against GM Alex Yermolinsky (Annotated Game #4) for an illustration of how the opening can be prepared (and how the endgame should not be played) in this line.
- Budapest Gambit (1. d4 Nf6 2. c4 e5). This whole opening is designed to be aggressive and trappy and is not easily dealt with by White. In my tournament career, I've more than once been seated at a board next to a game where this gambit had been played and personally observed Black obtaining a won game in less than 10 moves, while White was completely bewildered by what had happened.
Goal #2 (reach an at least roughly equal position) is more flexible but also requires more judgment and evaluation. With non-tactical sidelines or offbeat openings where there is no immediate threat from your opponent, sometimes just a basic idea of how to proceed can be enough, or perhaps knowing a sequence of 5-6 moves will set you up well for the middlegame after some additional standard developing moves. In more complex cases, especially in whatever the main line is considered to be, there is more danger of your opponent (especially when you are Black) being able to emerge from the opening with at least a slight advantage. Greater depth of move knowledge - and knowledge of why the moves are being made - is more critical here. Examples:
- 1. b4 (Sokolsky Opening). The standard advice I've seen is simply to play a Queen's Indian Defense setup against it as Black. This worked out well in the one game I've ever played against it.
- Colle System. I used to stop at move 3 with my preparation, which simply involved playing 3...Bf5 and then exchanging off the light-squared bishop. This seemed to take most of the potential sting out of the system. Following my game analysis here, my preparation has been extended out to move 5, for example as in Annotated Game #80, following the recognition from Annotated Games #75 and #78 that Black had to watch the b7-pawn more closely early on.
- Caro-Kann, Advance Variation. Most recently shown in Annotated Game #74, there are several early sidelines to know and the main line of the variation is considered the most challenging to Black.
The last big factor I take into account in determining "how much is enough?" is my ability to retain and understand opening lines - which, like everyone's, is limited. This naturally affects overall repertoire choice, choice of lines and also how far we will study a line until it is considered "enough". Take the Colle System example above: if I know it will be a more frequent choice of my opponents, then for practical purposes I will devote more time to looking at effective ideas for Black. However, I am satisfied that I can reach an equal position with my current knowledge level. So, in the absence of another motivating factor, I am able to safely concentrate on other openings. I've also deliberately limited the amount of theory to know in the Caro-Kann Advance by choosing the 3...c5 sideline, but it is still a critical variation to know and understand, so requires more attention during opening preparation time.
I look for practical results from opening preparation more than anything else and this approach has worked well, to the point where I can (and should) concentrate on other aspects of my game. It is also important to highlight that opening study methods can (and should) go far beyond identification and memorization of lines, if you want to truly improve your game.