How much music theory is required?

It varies. It's always good to know some. A simple example - if your music is in four flats and the score says "ad-lib solo", would you know where to start?

I play lead guitar for a Big Band - music by Glenn Miller, Rat Pack and lots of jazz (and no Tab). Two of us (at least) have music degrees, one of our woodwind desk can play sax, flute and clarinet to professional standard, and arranges music for the band dealing with all the transpositions - Bb clarinet, brass in A and so forth. I tell you, if I ever need to learn another instrument, I'm going to her for lessons. Our musical director doesn't write out chords, so when he arranges something he gives me a sheet with little black dots on the staff - I have 20 seconds before he gets back to the front of the band and picks up his baton to count us in, and then I have to do chord harmonies. No theory? You'd be stuffed. You'd be fired.

In a rock band, much of what you need to know is covered by I-IV-V, or I-vi-IV-V and one pentatonic shape out of five - is it any wonder that a documentary about (classical guitarist) John Williams back in the 1960s opened with the statement that you can churn out a rock guitarist in 20 hours. You're unlikely to be required to resolve a Neapolitan 6th if all you want to play is Metallica, but the more theory you understand the better.

A lot of rock guitarists never learn theory. As a result, their songs all sound the same and they usually disappear without trace after one or two albums - think Nu-metal bands.

The chord progression is simple and consistent in rock. People like Slash are pentatonic masters, but that limits what they can do note wise.

Books on music theory are not expensive. I recommend 1-2. Start with Music Theory for Dummies. Simple, easy to understand and doesn't so dumb it down as to insult the reader.

I also have the Hal Leonard Guitar Method Music Theory supplement to their Guitar Method book which includes audio for ear training. Not expensive.

Remember, knowledge doesn't hurt, but the lack of it may make your work much harder.

It's a really broad question and it depends on many things. I would say it's never enough music theory.
Yes, guitar is relatively simple compared to piano and you can start with no music theory whatsoever. Just memorize the chord progressions and you're good. There are tons of easy songs that anyone can play with a 5-minute preparation.
However, it's a whole different thing when it comes to writing your own music. To write a decent riff, solo or melody line you either need some knowledge of how chord and scales work together or just luck to come across that note combination that sounds nice and could be used as a riff for a song.
But again, is it a reliable method just to "hope" for it to just come? I wouldn't bet on such a band succeeding.
I personally think that all these rumors about rock guitarists not knowing theory is just a big bullshit used to raise their own self-esteem and show how cool and talented they are. In reality they do know chords, notes, scales, they did take guitar lessons at some point. It's just a matter of being honest and open about it (for example Buckethead - one of the best guitar players of all times - mentioned that he took lessons from Paul Gilbert). At the same time I hear some other "rock stars", who become popular more by PR and advertisement, claiming they are too talented and they don't need theory (when their music is a fucking progression of power chords that any punk kid can play).

So to answer your question, I think the very least theory to start with is: chords, scales, keys. It's just going to make your life easier and help you become better not only as a guitarist but also as a musician. At the end of the day, it's all about making music, guitar is just an instrument that helps you with this.

There was a link in alanguitar's post, and normally if there's a link to a third-party site in a first post we dis it as spam. However, the rest of the post made a contribution to the debate so I let it live.

The link didn't actually work, though, so I edited it out

alanguitar - if you own the site you linked to, talk to Paul (GN site owner) or David (Senior Editor) about paid advertising here.