I've been trying to suss out this little gem for a few days... but I think I've finally got it.
According to the skill descriptions, once you finish Better Learning 5, you should have a 20% reduction in times and not see the diminishing returns until 37 skills - exclusive of Better Learning and Unlearning.
I started off learning all of the BL and then the UL, then started moving into other skills. On two accounts, I'm seeing the compounding start at the 33rd skill (if I take BL and UL out of the total count). I don't know if the Better Learning skills are getting added into the count or if the threshold is not translating correctly.
To be more clear... I've learned Better Learning 5, Unlearning 3 + 32 other skills. When I started learning the next skill, my times went up by 3%. For example: Cocktail Crafting 2 has a base learning time of 5 hours, reduced 20% to 4 hours. When I started learning my 33rd (41st if you count BL and UL) skill the time went up to 4 hours, 7 minutes, 12 second... or Base + 3% - 20%. If I learn another skill that becomes Base(1.03^2) - 20%.
So essentially I'm getting the time reduction, but not the benefit of the 5 extra skills.