Inspiration comes from a lot of sources. I've got a story on another site that's an attempt at writing my wife's favorite fantasy, most of my m/m stuff is flights of fancy drawn from relationships in my own life, and I've got a couple higher-concept pieces I work on that were inspired by something I was reading. I've got one set in Ancient Rome that I really like and probably won't ever finish, since it seems to want to be a novel.
As to time, my latest story is the first where I kept track. My early stuff, including the stuff I put here four years ago, was just the first scratches of my learning to write and I probably don't want to know how long they took. But with "Samuel" I wrote about five pages a day to a total of 36 pages. Then it took a couple read-throughs to correct style and grammatical issues. I tend to get stuck on certain phrases and repeat them too many times, and it's a pain in the ass to edit that kind of stuff. Next step was cutting for length. I dropped about seven pages, in the end. Still not sure if it became a tighter story or if I just scrapped some of the tension, but I'm still learning. And finally, when I felt it ready, I ended up altering or adding paragraphs after each of the sexual encounters to form individual chapters so I could post it here, since it was well above the 10k word count. All told it was about forty hours of work. I have some hope that at some point in the near future I'll be able to put together stories of similar length and character development in thirty hours or so.
I'm also hoping to learn to put together a single shorter story in one week's writing time, which is about fifteen hours for me. That might qualify as hoping to grow wings, but we'll see.
One can see my two biggest problems right here in this reply. One: Verbosity. Look how many words I just spent on this. And two: Self-discipline. I'm "working on" the next chapter of "Samuel" right this instant. Except I'm not, I'm really writing two sentences and then browsing the internet.