First, let's make a difference between knowing how to play in the opening and opening theory.
You need to know how to play the opening. At first this means you need to know what your goal is in the opening (development, king safety, center control, preventing your opponent from reaching the same) and what that translates to in actual positions. Then you need to have some plan of which openings you want to play, and you must know what the strategic goals of each are, how to play them. Otherwise it's going to be hard to find good moves. After any serious game you should analyze it and figure out what went right and what to do differently next time, and that is true for the opening just as much as for the rest of the game.
On the other hand, opening theory is the almost scientific search for a White advantage. It is about specific lines and once it has been demonstrated that black can equalize in a certain line, opening theory stops being interested. Until the day that someone tries a move that hadn't been considered before and that does lead to a White advantage, until Black's answer to it is found... There are mountains of theory and it's impossible to know it all. If you know a lot of it, and you get it on the board, and it happens to be a line that Black hasn't equalized in yet, you might get an advantage. Which will be useless unless you are strong enough to convert that advantage into a win.
It can be fun to follow opening theory, and if you learn it you can play the first moves like the top GMs do and that is fun, but I don't think knowing a lot of it helps your chess results at all.
For instance, take this position:
1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 Be7 5.cxd5 exd5
Theory is not interested in this position, as it is a version of the QGD Exchange in which Black has it relatively easy, as he will soon be able to play ...Bf5 and equalize. Books on opening theory will tell you that if White wants to play the Exchange Variation, he should play 4.cxd5 and hold back the king's knight for now.
Sadler in his book on the QGD however notes that it is extremely important to understand positions like this really well, because he used to be beaten in these equal positions as Black again and again "as a young IM". Understanding normal positions is the key to understanding an opening. But they're not interesting to theory.
Let's follow Magnus Carlsen. He played 1.e4 e5 2.Nf3 Nc6 3.c3 a few days ago, the Ponziani. It is generally ignored by theory since Black has many ways to equalize. Then he just played chess, and eventually beat the #5 in the world. That's a lot of hours of studying theory saved.
Work to understand the normal positions in your chosen openings, by playing through instructive games, playing them and analyzing them a lot. Don't memorize theory.