RE: What curation reward time frames count after HF20?
First of all, there was no splitting in 0-3, 3-5, 5-15, 15-30, or later even before the HF. What we have is the so called "reverse auction time", which was 30 minutes in HF19 and is now 15 minutes in HF20. During that time, the fraction of curation rewards that are payed out to curators increases linearly from no rewards at post creation time to full payout of the corresponding share after the reverse auction time. In HF19, the share of curation rewards that was not payed out was given to the author. Now with HF20, these rewards aren't payed out to anybody.
You can get curation rewards if you vote before 15 minutes - and you probably should vote before 15 mins in most cases if you want to get a good share of the curation rewards. What matters is the total value of votes before your vote, the voting time if within the reverse auction window, and the total value of votes coming after yours.
Wow, thanks for this answer @crokkon! This totally surprised me, and I have already given this a small try, and will try it further in the coming days to discover the best way to get nice curation awards! Again, thanks a lot for this answer, the best I have received to the question so far!
I'm glad it helps! If you're interested in more theory behind, see an older post here. This is still from HF19, so think of each occurrence of "30 minutes" as "15 minutes" to get a recent picture with HF20.