Because I didn’t know if I would feel up to trying the challenge, I didn’t make any announcement post. But now that I have actually completed it, I wish to add a post with my results.
My 24-hour time frame was Saturday 12:45 to Sunday 12:45.
The “book” I picked was the visual novel 薄桜鬼 真改 風華伝. I’ve been reading it for a while now and picked it for this challenge to make more noticeable progress.
I can’t give an accurate word or character progress number as I’m reading the physical Switch version. That’s why I tried making a rough guess and got 22’550 characters out of my calculations which I added to the results below.
Explanation behind my logic for the character count
When I look at the bare minimum of one word reactions/comments/responses such as「…」, 「えっ？」,「NNさん」, 「そうですか。」and the like for one text box which span anything between 3 up to 10 characters, I’ll assume a minimum average of 5 characters for one text box to keep it simple.
Whereas the maximum box capacity for each line – 3 lines are the maximum per box – might be 25 characters each, I rarely saw that happening. Usually, it’s about 15 characters for each line if the box is filled with 3 lines, so that would mean an average of 45 characters.
Therefore, I’ll use this average minimum and average maximum and divide it by 2 for the average characters per box → 5+45/2 = 25 characters
I ended up using these presumed 25 characters for my estimation. If I multiply the 902 textboxes by 25, I get 22’550 characters.
Again, this is a very rough estimation since it could be off by several thousands of characters but it’s the only possibility I have without going down a rabbit hole and trying to find more chapter specific numbers online which I don’t fancy doing.
I don’t know if my logic makes sense (wouldn’t be the first time that my math isn’t math-ing) but I hope that this number is acceptable.
Final result 
Time spent reading: 6 hours and 37 minutes
Progress estimate: 5% of chapter 3 → 30-ish% of chapter 3
Textbox count: 902
Character count estimate: 22’550
Thank you very much for organising the Readathon, @taiyousea! 
Also, it’s really interesting and inspiring to see everyone’s thoughts, results and progress updates so far. I hope that everyone who isn’t done with the challenge yet keeps having a good time. 