Dialogue

It depends on the book. Some books are better with tons of dialogue, others need more in between.
I don't know how to say how much is "too much" because for me, too much would just be if I felt like I didn't know who was speaking at a given time, or if I felt like the dialogue was telling me things I should be learning through action, or something else like that. If it's working, it's working.

When the reader loses their grounding in the story because there's nothing but dialog, that's too much. When the reader has to slog through dialog that doesn't move the story along in some way, that's too much. It can't just be heads talking in a white room of nothingness.

If the dialog moves the story and you also have enough other stuff so the reader can stay grounded in the story and understand what's going on, then you've found the right balance, whatever that is.

It has to be meaningful dialogue, something that moves the story. That doesn't mean you shouldn't have your characters not talk about any other thing that isn't connected to the main plot. You can have two characters talk about something random, to be quite honest. You just have to not go overboard with it. Frankly, I think it's weird when I read a book and all the characters ever talk about is something to do with the plot, especially when they're YA characters. They don't discuss any other thing. They don't talk about their hair, or that party, or something that young adults would normally talk about, and I think that's weird. Of course, it won't be weird if your characters exist in a world that doesn't really allow for talking about frivolous things, like The Hunger Games: I don't remember Katnis discussing hair with anyone. She just thinks about it for like one second and that's it. She's off to facing the problem at hand - the capitol.

But then you have Harry Potter, where the characters talk about a host of different things: Christmas, presents, girls, school work, etc.

The trick is to find balance, mate

I come from a theatre background, so dialogue is usually one of my favorite things to write. That being said, it's all about finding a balance, and I struggled with that for a long time.

Dialogue should always move along the story or the characters. Like Glenakin said, I find it really weird when the only conversations we get to see are directly connected to the plot. I love characters, for me that's what really makes the story, so I like seeing the dialogue, as long as it doesn't somehow becoming the characters telling me everything I need to know.

Dialogue can be action, if handled correctly. Also, don't forget subtext - pages of seemingly irrelevant dialogue can grip like a vice if you know what's really going on between your characters.

Also remember that plot is different from story. Plot is the stuff that happens to illustrate the story. Story is why it happens at all. So you can move the story along without talking about plot, because the dialog might develop a character more or lay some hints of events to come or do any number of others things. It might not be plot, but it is story.