How long do I want my novel to be?

This may sound like a weird question, but I am starting to wonder just how long I want this novel to be. The more I edit and revise, flush out and refine, the longer this novel seems to grow. It truly is starting to take on a life of its own.

Many of the characters and scenes have even started sneaking their way into my dreams. Which, actually, has been a big help since when I wake up I think to myself "Wait a minute, it worked out much better in my dream than the way I wrote it" and gives me a sort of fresh look on this scene.

And while I realize that there's no set formula for determining the length of your novel, I'm just starting to wonder how long people - in today's world of iPads, iPhones, and iBooks - will sit and read a real book. And how marketable is a book if there is no market for it?

One site I read regarding writing a novel mentioned that an average length to shoot for when writing a novel was 70,000 words. Well, pfft... NaNoWriMo is 50,000 words, and they call it a novella. My novel, as of right now, is roughly 122,000 give or take a chapter heading.

And perhaps I should mention that this is just book one of a trilogy? It just started to grow so long that I took my outline of ideas and drew two lines through it. Now, however, even each section of my outline is outgrowing me. How do you know when it's getting close to that line of "this is long enough". It seems to me that a good novel is never quite finished... I can come up with questions and loose ends on nearly every book I've ever read.

So how do you wrap up those loose ends, tie together those plot twists, and answer those questions without drawing out the novel too far?