I hope this isn't too detailed... I'm lucky that story ideas come to me quickly, and that I write original drafts quickly. I always look at the Challenge photo on the 1st day, let its essence sink in a bit, then just go about my daily chores. At some point - almost always within a day or so of seeing the photo - an idea comes to me, a central concept for a story to be formed around. Then I let that settle for an hour or so, and the whole story comes to me. I write it down on paper over 15 minutes or so... and it's usually pretty much what I enter except for, often, two things ... a new, better closing line will come to me (the original sometimes being just a placeholder which allows me to write up a completed story), or a new, better title comes to me.
The most time for me, by far, is spent editing down to 300 words (well, I try for 298, to be positive I won't go over the word count; I'd say my original drafts are around 500 words, on average).
This all happens usually within the the first 3-4 days of the Challenge. Then I just pray for the 10th to come, because I always have a tendency to tinker ... I might re-order a paragraph, or change one word 2-3 times to make a sentence more effective. On the 10th, I can dump, er,
post the story, and forget about it.
Then I read the other stories, and wish I'd taken a bit more time.
edit - overall, I'd say it takes me on average 3-4 hours for each 300 worder. The Challenges are about all I write, so I might take a bit more time than some of the folks here.