i finished japanese tree (android) in ten days.it was funny.but i have to say i already know japanese. The vocalization of the words could be better.i get it it must be hard but even though it doesnt matter because you are duolingo.the placement of the words must contain onyomi and kunyomi vocalization in the right way. I LOVE DUOLİNGO
I did notice the length as well. Compared to FRENCH the JAPANESE is extremely short. I wish they focus a bit more on GRAMMAR, hopefully the web version will jump deeper into GRAMMAR. As for the vocabs, there has been a few scenerios where it doesnt make sense. Then again, Japanese to english and vice versa (literal translation) doesn't even make sense.
I hope they increase the amount of lessons in Japanese tree. I enjoy they're work, hopefully by having ads, they'll scale they're trees bigger.