Okay, got through the rest of the first (of 3) "making of" videos.
Narrator made a comment about how they were new to band, which reenforces what has come before.
They practiced in the morning and, apparently, worked on their acting roles in the afternoon. Nothing about how long they practiced, but based on my experience and how intense this had to be, I'd guess a minimum of 2 hours and quite likely 3. Definitely would be a killer on the lips which explains holding towels up to them!
When filming starts, you might remember the "making of" scene where the 2nd trumpet player pushes her arm out in front of her in prep for the scene. Apparently she is told not to practice it - just do it.
That's a small tihng, but I think it fits the pattern of the movie that the director wanted things to be "natural" - thus teaching kids from scratch.
During filming, they were still practicing in the morning.
During the filming for the final concert scene (starts around the 21 minute mark), my friend said there are comments about the music for "the whole movie by themselves" and "did it themselves".
Since this is the most critical thing for me to absolutely confirm, I asked him to have his Japanese wife listen to that portion.
It was mentioned a couple times that they worked 10 months. He did point out that it could be interpreted different ways. For example, they could start at the end of one month and end the beginning of the 10th month and still call it 10 months. There is also the question of whether they counted time between the warm weather scenes and resuming in the snow.
Anyway, based on what I have so far, filming started July 25, 2004. The music teaching begane 3 months before so, guestimate about April 25th. Winter filming resumed February 17, 2004. That would be just 8 day short of 10 full months and certainly they wouldn't have finished in one day! So figure late February or early March for finishing.
Unless they did things out of order on the "making of vid", the last scene was done before winter scenes. No idea how long that took. We just know about the initial 3 months and that practicing did continue during filming. Odds would be that the last concert would be at the end of all that, just to give them as much time to develop as possible.
But with that scene out of the way, it would mean they probably had a LOT of time away from playing before they did their live concerts!