It's been a while since I dedicated myself to learning Japanese in the way I probably should. I remember years back when I would study at least an hour daily -- and the progress was so noticeable. It was a constant sense of progress, and it was pretty awesome picking up a novel and seeing the kanji I had just learned pop up.
I'll be visiting Japan in the next few months and it's made me try to pick up studying a bit again. Building that habit again is a challenge, but like you said -- it's a daily effort!
Yeah, I'm studying at least that much right now. I think that even if you don't end up doing anything new for the day, you should at least doing your review cards (if you're using SRS/flashcards, which you should be). That is what I do on Niko's suggestion and it keeps you from falling behind while doing something that it somewhat less intensive.
Have a good time in Japan! I had a great time when I was there a few months ago and I'm going to be going back again in the near future unless something changes to prevent it. Maybe I'll see ya there ;P