Good question (albeit one I’m coming to super late).
It kind of…depends on what I’m writing. For the Behind the Scenes chapters, I can usually knock them out in a couple of hours, and, as they all (more or less) fit around the show, I don’t need to think about them too much in terms of plotting. The one exception was…I can’t remember which number, but the chapter covering Dan’s point of view in season 6, partly because it was LONG – most of the BtS are quick snippets, but I had to cover a lot of ground in that one – and partly because I really needed to think through where he was emotionally at every point of the season before I could write it. And, as much as Dan is shallow emotionally, there was still a journey to be captured there.
But some of the other fics require a bit more planning in advance… To Wise to Woo Peaceably was a bit of a monster, and I had to map it out before I could write it – chiefly because I always knew that Brie was going to play a crucial role towards the end of the story, and I had to work out what, if anything, she would see that would allow her to do that. Similarly, I held off on writing a season 7 fic until I had a very clear idea of…how the various moving parts would all fit together. There’s more plot in both those stories, so they need to be worked out.
In terms of writing time – it varies a bit. The Too Wise to Woo chapters ended up being VERY long, because there was so much ground to cover – and I’m not one of those sensible people who writes well in advance of posting. If I’ve posted it, I’ve usually only finished writing it a few hours before. I tend to upload it to A03 and then give it a quick proof read before publishing, but there’s not usually much in the way of revision. I’m fortunate in that I’m not overly prone to typos, though you can usually assume at least one or two will slip in. I seem to remember I read the first chapter of Best Laid Plans over several times, but that was chiefly because I wanted to be sure the time jumps were reasonably easy to follow.
The easiest part, in terms of writing, is the dialogue – just writing them bantering back and forth is a lot of fun – but lordy do I hate writing sex scenes. In fact, I try to skip writing them unless I consider them crucial to the plot, because they are so hard to write well and almost impossible to write distinctively. They’re probably the thing I get most nervous about.
All of this pales in comparison to what it’s like to write a play, mind you. I can knock off a chapter of a fic in a day or two at the most, but plays…plays take a LOT longer, and a lot more…intellectual work before I put pen to paper.