@Martin_Stahl, I think we can all agree to the point that learning obscure opening lines hidden in strange variants won't profit anyone. Except you're facing an opponent who's known of playing exact that obscure line.
But on the other hand, wouldn't you explain to a beginner the advantages of e4 compare to a3 and maybe go to the Italian tho show them how to apply the opening principles? Where is the line between "just enough" to understand the principles and "to much" theory?
A fitting quote from one of my favourite philosophers, Immanuel Kant:
Gedanken ohne Inhalt sind leer, Anschauungen ohne Begriffe sind blind.
(roughly translated: Thoughts without experience are empty, experiences without thoughts are blind.)
But opening principles, in most cases, will cover the needs of that. 3-4 moves in any opening, with some basic ideas, is about as far as most players are going to get before they get a position they don't recall or never studied. At each step of the way, there are multiple paths and trying to learn about all the possibilities that are playable, quickly becomes unwieldy without a great memory.
Knowing where that line is, will depend on the person some but I wouldn't expect most player to get very much benefit spending more than around 10% of their time on openings, with the exception in post game analysis looking for ideas that just don't work and figuring out why and what would have been better and still suited to the way the player thinks and plays.
I have to say I disagree. I can bang off a list of opening principles that you could memorize almost immediately, but they're pretty much useless unless you learn to apply them practically, and the same can be said of opening theory. Learning a bit of theory isn't very hard and isn't very time consuming. As I've said before, you can learn the basics of the Caro Kann in an hour, and if you wanted, you could play it for the rest of your life. Firouzja uses it in almost every tournament that he plays in, for example, and I hear he's pretty good. Additionally, you don't have to learn it all at once.
You also make the somewhat unreasonable assumption that you can somehow *study* how to not make mistakes, and you assume that things like doing puzzles first, have the same kind of one-to-one correspondence in time spent to learning. Does an hour of doing puzzles help more than an hour of doing some theory work? I tend to doubt it. I've done zillions of puzzles and they don't help much. As far as I know, there is NO surefire way of learning how not to blunder or how not to hang pieces and this all comes with playing games. Does playing three games of rapid improve you more than an hour spent on theory? I doubt it. Does analyzing two games for thirty minutes each help more than an hour of studying theory? Maybe, though maybe not. Most of the times when beginners lose a game it's because, surprise, they hung some pieces. Does analyzing games help with that? Not really. Analysis doesn't help with board blindness.
So once again, you're assuming that time is better spent elsewhere and that there's a one-to-one tradeoff. I wouldn't be so sure and it's an awfully big assumption that such a tradeoff exists.
I certainly used to study openings more and have a plan on working through my current opening issues but I'm going to tie it more in to more patterns and plans, along with finding the places where my ideas just don't work. However, I can verify from both personal experience and from looking at the analysis of other people's games (both here and OTB), that the opening doesn't make a ton of difference in most cases.
As can be shown in this topic, there are coaches and masters that suggest not to worry too much about openings (beyond a few moves, principles, and maybe playing the same openings over and over to get familiar with the positions) and some that do. A lot of people put a lot of stock in opening study and while some do well with it. A lot of others find themselves floundering, swapping openings all the time when they're still not getting the wins they want, and ultimately the problem isn't the opening at all.
The two places I mostly see the proponents for opening study are those well below the rating where they benefit much from it (sub-1200) and those were it begins to make more sense to do it 1800+. The higher the rating, the more likely you're going to be facing people that know openings pretty well and getting to the middlegame with that knowledge actually may make a difference. Those in the middle ratings shouldn't have a major focus on openings and I still think organically growing the opening knowledge (or having a coach point out issues) is more beneficial in the long run.
I've made it mid-1600's US Chess on what study I've done, though have fallen a few times. I know the basics of a lot of different openings and in general I'm getting into early middlegames most of the time without major problems (though there are times when that's not true). It's from that point on where the problems start and I'm missing tactical ideas and having endgame problems. The same is true of other player's games I've looked at.
There's a local player in our club that has made it up to over 1700 US Chess playing a lot of sub-optimal openings (not theory) and he is able to play at that level because he's fairly decent at tactics, endgames, and very good about creating complications (even if they may not be the best).
With my bad rating, you don't have to agree with me. Just ask yourself if your opening study is really helping you that much. I think you'll find that if you analyze your games, you're missing tactical ideas, missing your opponent's blunders, not playing the endgame as well as you could, and may have positional problems. I know that's true for me