That's a question that all chess players need to answer, and it will be different for each of us based on a number of factors:
- what level are you at?
- what style of opening do you choose to play?
- what ambitions do you have?
So the level that one is at will determine how deeply a player needs to study openings. The higher your level, the more you will need to understand openings, as you'll probably be playing players of a higher level.
The style of opening is important because some openings have more tactical lines, while some require more positional understanding of structures and ideas. Of course all openings have tactics, and positional ideas, but certain openings tend towards one or the other.
Your own goals will influence how deeply you work on the game generally and openings specifically. I am not particularly ambitious having got to where I want to go, so I tend to not put much work into the game (or at least not the sort of work that might help me improve). However, a player who wants to jump 200 rating points might work on strengthening their opening knowledge as well as on other parts of the game.
The main thing to do is assess where you are at, what you want out of chess, and to make a plan going forward. Do not try to emulate the professionals unless you are close to their level, or thinking of becoming a pro. (If you do have these ambitions check out the new pro chess league at chess.com).
I saw a game that made me feel odd about opening study. It was like, "why bother working on the game when this is where it will take you?" The game was Mamedyarov-Gelfand from the Rapid tournament in Tashkent. The game followed So-Giri Leuven 2016 all the way, ending in exactly the same 3 move repetition. I'll admit it is a fascinating opening, black sacrifices a piece and gets very interesting play for it.
The Semi-Slav has been a hot opening since the 1990's, and even I've played it! A topical line from the above position is 12.b4 making black's c-pawn very backward. Although it was first played in 1998, the move has been played more often since about 2012 when it was played in some top games, most notably Topalov-Kasimdzhanov London 2012. 12..a5 13.Rb1
Here, the most drastic way of dealing with the backward pawn is to ditch it with 13..c5. After 14.bxc5 Bxf3 15.gxf3 black has created weaknesses around the black king, particularly on h2 and g2. It isn't enough to win but seems good enough to offer equal chances. There are quite a few perpetual finishes in this variation.
So in this position, Giri played 15..Nxc5 sacrificing a piece for a pawn. Black gets a useful pin on the c-file, and the ability to transfer his rook and/or queen to the king side. After the sequence 16.dxc5 Rxc5 17.Rxb5, white regained the pawn to be a whole piece up, and has guarded a black rook swing on the fifth rank.
So now black sacrifices a rook to gain a perpetual. 17..Nd5 18.Rxc5 Qg5+ 19.Kh1 Qh5
The threat of mate means that white must play 20.f4 when black grabs the draw by Qf3-g4-f3-g4 etc.
It's a fascinating variation with some very interesting positions, especially concerning compensation for sacrificed material. There are also some very interesting endgames that occur, when white refuses to take all the material offered. I can understand why Giri played the variation earlier in the year, as there are ways for white to go wrong, while he had a cast iron draw available. What I can't understand is why anyone would repeat it. I guess that is why I am an not a professional chess player!
Anyway, here is the Mamedyarov-Gelfand game, and I've also built a file with all the games that start with the variation 12.b4. Enjoy!