How deep does the rabbit hole go?

Curiosity.

It's a natural and very intrinsic part of human nature. The five W's (Who, what, when,where,why) and the honorary H(how) are intrinsic part of our daily lives. Try and guess how many times in a day you ask questions, either of others or yourself. They range from "How do I work this infernal device?" To, "Why are we here? What is the meaning of life?"

The clearest indicator of this universal trait is children. I've lost track of how many times my mind was on the edge of something brilliant only to be interrupted by "Mommy, why did that guy on the movie last night..." You get the idea. Their favorite tends to be why, a constant headache for parents. Why is the sky blue? Why do I have to brush my teeth? I try answer them to the best of my abilities whether to relay information I already have or to later look it up with them.

I try and pass this on in my characters, to make them believable by having them ask questions. Even if the question is a rhetorical "Why the hell am I doing this?"
or a snide "What is wrong with you?"

Here are some questions for you. How far should I take it? When does the ask and answer get tired? When do you, the reader get sick of listening to one character berate another for lack of knowledge? How much back story do you want?

The reason I ask is because I'm currently in the throes of my Laundry Hag's back story. Or at least it started out as back story but it's becoming a stand alone. The questions I set out to answer, How did Maggie and Neil first meet? What brought them together? When did Leopold come into the picture? Who is Josh and Kenny's birth mother and why isn't she in the picture? What was Maggie's childhood like and how did she become the character that I know and love? Yet for every question I answer, five more pop up.

So tell me, what do you think? Where should I draw the line?