I think this is one of the hardest things to get right in a story. Or maybe I think this because I'm working on it right now and feel like my head is about to explode. :)
So, what is Voice? Basically, it's what you hear in your head as you're reading a story. If it's authentic, it'll be obvious that the main character (MC) is telling the story. Every paragraph, every sentence, every word is exactly what MC would say. If it sounds generic or jarring, then MC's voice is not coming through.
My next question is this: can Voice be taken too far? Can MC be inserted so much into the story that it slows things down? I think it can. I think it's extremely hard to find that balance between hearing MC's voice and MC hijacking the story.
But it's MC's story! How can she hijack her own story?
Technically, yes, it is her story. But I believe there are times when MC can get in the way. Particularly in today's YA books. I've read quite a few that have lots of inserted personality, especially in the beginning. Granted, this is a good thing because we get to know MC right away. And every piece of information, pondering note, and bit of backstory was clearly in MC's Voice. But it was sometimes so long that I found myself saying "get on with it, already." I've even skimmed through stuff like this, potentially missing important information, just to get to the next bit of dialog.
Of course, not all books are like this. I think SPEAK by Laurie Halse Anderson gets Voice across quite clearly. Every single word is a reflection of the main character, Melinda, yet the story is still moving forward. THE STORY OF A GIRL by Sara Zarr is another example. But there are other stories that have half a page of pondering or expostion right in the middle of a conversation. I mean, who can go through all those thoughts, simultaneously think up a reply to the conversation, and keep the other party from looking at her funny? No one I know.
So, how much is too much? To be honest, I can only answer that from a Reader's perspective. If my mind starts to wander, it's too much. Subjective, I know. :) I guess a good way to handle this is to find someone with a similar threshold and ask him to read the story. Based on his reaction, we can add/edit/delete as needed.