williamt2214-san, マイケルsan,
I agree with マイケルsan; even if it's only a little time, studying everyday is the most important thing.
As far as I'm concerned, 2 or 3 lessons in one day is fine though
But it might also depend on how you find those lessons. If you find a lesson difficult, probably it's better to
listen the same lesson twice or 3 times, instead of going to the next lesson.
I've done that before (with different languages, obviously...) and it works very good.
On the contrary, if you find a lesson easy and you can learn a bit more, then go to next lesson and get more new stuff.
Regarding concentration, it's impossible to be concentrated 100% if you're doing something else at the
same time (anything, really). One good milestone about real concentration would be the time 1 interpreter (simultaneous interpretation) can go on, and that's only 15-20 minutes max. If you imagine how concentrated those
interpreters have to be, you might know you can go on longer than that when you study.
When we study, 5 or 10 minutes break after 60-90 minutes are considered good and efficient.
Hope this helps!
Natsuko (奈津子),
Team JapanesePod101.com