NaNoWriMo. The idea wasn't developed very well at that point, but it was a start.
Over the next few years, I changed the structure, format and overall idea of the coffee house book until it became, a few years ago, what it is today: the first book in the collection.
In all that time, as I worked on the coffee house book as well as other projects, had babies, took jobs, took other jobs, moved.... the coffee house book remained.
My mind worked on plot details, I saw characters and their relationships in people on the street, and whatever else I wrote, I always ended up back at the coffee house book.
It wouldn't leave me alone.
That's how I know the collection is where I need to be. Yes, it's an intense project, and will be incredibly time-consuming. In fact, it will likely take many, many years to complete. But I'm willing to take that time for it because it will be worth it. These characters' stories are begging to be told. They're important, and I can't ignore that.
I'm making good progress on the work now, and I'm looking forward to when the first draft will be done. Until then, I'll keep plugging along because my characters and their stories certainly won't leave me alone until they've been given voice.