How deep does the kings indian attack go
I'm not sure why this hasn't been made explicit yet, but it's obvious from both context and subsequent posts that the OP meant "KID" rather than "KIA." He just didn't know the difference.
And the answer to "how deep does it go?" is VERY. It's one of the most theory-heavy of all mainstream d4 defenses, and probably the single most popular club opening against d4. Beyond about 1500, everyone will be prepared against it.
It's still a good opening. But yeah, people know it.
Also, I find the suggestion that, "in the Sicilian, the KIA becomes the closed," to be one of the oddest comments I've ever seen from a highly-rated player on any site. Very strange.
The famous trainer Dvoretsky, author of many very well known books, has the opinion that the KIA is a great system of which to base one's repertoire on. He even dedicated an entire chapter to the KIA in his book 'Opening Preparations'.
Dvorestsky only advocates the KIA against the French and against Sicilians where black plays 2...e6.
He strongly advocates you play openings that suit your temperament, preferring this even over openings that suit your strengths as a chess player, the assumption being that if you're going to continue to improve at chess, you will invariably come to prefer the kinds of positions that best suit either your quiet or your aggressive temperament.

Does my preference for playing Queen's Gambit and the Ng5 response to the Two Knights Defence indicate that I'm crafty and always wanting to "trap" people? I really only play them because they're the openings that give me a fairly solid position even if they're refuted.
The KIA is discussed in Winning Chess Openings by GM Yasser Seirawan (1999).
https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf
For more on the KIA, one could try The King's Indian Attack: Move by Move by Grandmaster Neil McDonald (2014).
https://www.newinchess.com/media/wysiwyg/product_pdf/7277.pdf
Perhaps, it would be of interest to look at The Fianchetto Solution by Emmanuel Neiman and Samy Shoker (2016)
https://www.newinchess.com/media/wysiwyg/product_pdf/9029.pdf
and Starting Out: King's Indian Attack by John Emms (2005).
https://web.archive.org/web/20140627034051/http://www.chesscafe.com/text/hansen81.pdf
Obviously, there is some "theory" (stuff one can study), but perhaps considerably less than what is involved for many other openings. I suspect that many chess clubs have a KIA enthusiast.
"Average lines go 25 moves or so. Sometimes people memorize whole games which are 40 moves long." - BronsteinPawn
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)

Something of a problem? I thought it was about memorizing entire games.
Your constant thread trolling is a certain problem. Not?

... Fischer plays the KIA ...
Not very often, these days.
How do you know? Did you meet him last week?
... Fischer plays the KIA ...
Not very often, these days.
How do you know? Did you meet him last week?
I guess it has to be admitted that database records may not tell the whole story.

"Average lines go 25 moves or so. Sometimes people memorize whole games which are 40 moves long." - BronsteinPawn
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
The guy answered a question which required numbers. I gave it to him. I dont really give two pawns about what 2 homeless FMs that won their title in a kids event have to say about the topic.
Seriously, for low rated players the FM advice makes sense, when you hit 2000 FIDE I seriously doubt you can just get away with random plans, you dont need to memorize yet but you do need to dig deeper. When you are on Magnus Carlsen level you definitely need to memorize.
Radjabov would be laughing at those FMs, I am sure he memorizes quite a lot of stuff when he plays the Kings Indian.
I've been playing chess for a few months, studying it, practicing frequently. I recent saw the kings indian attack, and I instantly fell in love with how each peice can not only defend, but replace other peices, and flow into attacks. ... How deep to I need to understand this opening to use it effectivly. It seems pretty straightforward, but I'd like to know some ideas black might try to employ, and some squares they might try to capitalize on, maybe a few good games to study.
"Average lines go 25 moves or so. Sometimes people memorize whole games which are 40 moves long." - BronsteinPawn
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... Seriously, for low rated players the FM advice makes sense, when you hit 2000 FIDE I seriously doubt you can just get away with random plans, you dont need to memorize yet but you do need to dig deeper. When you are on Magnus Carlsen level you definitely need to memorize. Radjabov would be laughing at those FMs, I am sure he memorizes quite a lot of stuff when he plays the Kings Indian." - BronsteinPawn
I'm not sure that we have a disagreement here. FM Steve Giddins was writing about the needs of "the average player". Does it seem likely that sgt_pepper was very concerned about what happens when one hits 2000 FIDE, etc.?

The average player is actually 2100 FIDE.
The average chess.com player isnt.
SO WE ARE AT A DISAGREEMENT
To clear things up:
1. The KIA against the Sicilian is not a 'Closed Sicilian'. In the Closed Sicilian, white puts his knight on c3, while in the KIA, white hardly ever puts his knight on c3. Instead, a pawn usually emerges to c3.
In the Closed Sicilian, white hopes for a kingside attack, the KIA is much more flexible. Sure you can play for a kingside attack (e.g. e4-e5 against 1.e4 c5 2.Nf3 e6 3.d, or a plan of Nf3-h4 with the idea of f2-f4), but white can also direct play in the centre with c2-c3 with the idea of d2-d4, or if black stops that idea with e7-e5, a queenside expansion plan of forcing through b2-b4 with the help of the a pawn and the c pawn.
2. You can get a KIA from 1.Nf3. Playing the KIA from both the move order 1.Nf3 and 1.e4, I still think the move order with 1.Nf3 is the 'pure' KIA.
Some comments state that after 1.Nf3 d5, white is forced to play a queen pawn game. That is not true. After 1...d5, white simply plays a KID reversed, which falls under the name of 'KIA'.
Playing the KIA through the move order 1.Nf3 is not a Reti. In the Reti, white plays for c2-c4, in the KIA, white plays for e2-e4.
3. The comment saying that the KIA will never be successful shows that the one who posted the comment clearly do not understand the KIA. Fischer plays the KIA and produced some spectacular fireworks with it.
The famous trainer Dvoretsky, author of many very well known books, has the opinion that the KIA is a great system of which to base one's repertoire on. He even dedicated an entire chapter to the KIA in his book 'Opening Preparations'.
4. You can force through the e2-e4 push with 1.Nf3, black can delay it, but not prevent it. Please provide a line where black can actually prevent this advance.