Hi, Sorry I missed answering this one. Sometimes I just zip through the forum.... oops. But hopefully the answer is still appropriate:
The idea that you can play two hours of scales and arps without stopping for two hours, is a great goal for anyone who is a concert artist (solo, orchestral, jazz, etc), because most performances are at least an hour. If you can, with anything, go beyond the requirement of the professional gig, you will have the stamina to perform at your peak for the entire time. The goal is to play for two hours so that concert performances do not tire you. With this said, how you structure the two hours is up to you. It may take TIME (weeks months) to get up to a continuous two hours of scales and arps. I usually include improvisation, in scalar and arpeggiations, so that I do not get bored. Beware doing the same patterns every day as that can lead to complacency. One important factor is to be at HIGH AWARENESS and critique throughout these two hours. If you can concentrate that intensely, for that long, you can perform an hour concert easily.
Hope this helps.
Patrick
