Brewing, Composting, Percolating

I've been doing a fair bit of scribbling in my notebook. Nothing earth-shattering. Just processing ideas, writing about the pup, the kids, about me not writing productively (that is, no deadlines). A couple of days ago, a voice emerged. "This is my purpose," she said. And my hand is aching as this girl tells her purpose, or what she assumes is her purpose. It's a story that explores duty and freedom. I have been thinking of this for years and years and years. And finally, there's the voice, the perfect characters to explore this idea with.
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Almost all my stories, both short and long, have an extremely long gestation period. We're talking years here, not months. My historical YA has been percolating for a good twenty years. The novel I wrote last fall had been composting for a couple more decades. And this one -- I don't know what it is yet -- but I've been thinking about this in some way for close to thirty years. I'm in my 40s, so yes, I was a kid when I was first thinking about this. To finally have words to express these ideas is exhilarating. This is what I love about writing.
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So tell me, how long do your stories brew in your head before you actually start writing?
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