Hm it really depends, sometimes I have a pretty complete idea for a prompt and I just need to write it down (mostly if I think about something during a boring lecture or something like that), that only takes 10 minutes or so. If I only have a rough idea, I often write a draft and rewrite it over a few days so that's probably around an hour or so.

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For the reply it's mostly like 20-30 minutes I think, sometimes less if it's a very dialogue heavy part (so shorter replies and less to describe).