I do the quarter drill and the pinky drill at 5 minutes each. I think that is all I can do on those. Then, I do scales for 7 and 1/2 minutes, right now, D and E flat major. I play Wolfhardt etudes 7 1/2 minutes, and I work on a performance piece for 7 1/2 to 15 minutes. I know Mike says to divide into thirds, but the drills and the etudes tend to wear me out a bit.