Most people, from all the conversations I've had with my own readers and those on SH/RR Discord servers, will read a 1,000-word chapter in about 3-4 minutes at most. 3k-5k word chapters are usually finished in about 10-15 minutes at most.
Also worth considering: the average reading speed is about 300 words per minute, but this just a global average. Voracious readers who read a ton of web serials will read much, much faster. I know people who can read an entire 1k-word chapter while retaining all relevant information in under 1 minute.
It always kinda stings a little when something that takes 5 hours of work is completed by the readers in a few minutes, but that's just how it is.
EDIT: wait I completely misread the question. My bad. I'll leave that there for anybody who's curious though.
EDIT 2: To actually answer the main question, it takes me about 30 minutes per 1k words on average. My 3,000-word chapters take me 2 hours and my 5,000-word chapters take me 3 hours. The longer the chapter, the longer I'm in a good flow of writing for, so my writing speed goes up.