out of curiosity, i wanted to do some math to figure out how long volume 9 could potentially be despite it being a shorter volume episode-wise, and how it compares to volumes 5-8.
disclaimer: i'm not good at math, especially when time is involved. i used so many calculators that did most of the work for me, so hopefully there aren't too many mistakes. still, heads up.
so, here are the full volume lengths (episode count in parenthesis):
v5: 4h 6m 56s (14)
v6: 3h 54m 00s (13)
v7: 3h 59m 35s (13)
v8: 4h 26m 11s (14)
v5 average episode length is about 17.5 minutes, both v6 and v7 are about 18, and v8 is about 19. if volume 9 has 12 episodes and the same average episode length as volume 8, its full volume length would be about 3h 47m 59s.
that's 6 minutes shorter than volume 6 with one less episode.
of course, this is purely speculative: we don't know how long the episodes are going to be – they could end up shorter, or they could end up longer – but based on this and how the writers have said that they prefer the longer episodes... i really don't see volume 9 being drastically shorter timewise than previous volumes just because it has less episodes.