The answer is, however many fics you WANT to write at that stage, and bollocks to the concept of normal!
In terms of shows, I've written Manifest after one episode, Fringe after one (crossover with Lost which ended up taking me until the middle of season 3 to finish), From after three (although I'd caught up a bit more of S1 by the time of actually posting that one), Dark after about half of S1 (which mostly worked fine for my one shots although there is my ongoing series which I started writing before I fully understood the complexities of the family tree and realised that what I had done would never actually result in the happy ending I wanted for Mikkel). In other cases I have left it a lot longer.
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In terms of shows, I've written Manifest after one episode, Fringe after one (crossover with Lost which ended up taking me until the middle of season 3 to finish), From after three (although I'd caught up a bit more of S1 by the time of actually posting that one), Dark after about half of S1 (which mostly worked fine for my one shots although there is my ongoing series which I started writing before I fully understood the complexities of the family tree and realised that what I had done would never actually result in the happy ending I wanted for Mikkel). In other cases I have left it a lot longer.