American Fan:
My best guess...Older K-BLs are shorter, many 15 mins or less, maybe cuz of less popularity and budget. BL themes weren't exactly mainstream, maybe less so in SK.
But starting in 2023, I see more K-BL's 30-35 mins long per ep., maybe cuz it's gotten more popular/mainstream in SK?
It also seems like a few years back a lot BLs of any country only had about 6-8 eps per drama, but nowadays it's often 12-16, except for J-BL's.
I look more at total runtime. While I love Thai BLs, they're a big time investment, and there's a lot to be said for concise, tightly constructed KBLs and JBLs with a two- or maximum three-hour runtime. Example: "Live in Love" (not good, not sure why I'm watching it) is only five episodes, but with episode lengths of up to 90 minutes, it's still the usual eight-hour Thai marathon.
And then there's the recent trend (I think originally in Korea, now also in Japan) of "micro dramas", i.e. episodes of just a minute or two. Grrrr! I guess they're designed for busy, on-the-go people to watch for entertainment while they're waiting for the crosswalk light to change. That being said, some of them are pretty darn cute.