If a week seems like a long time to see how a student is progressing, try having the student videotape small portions of his practice. I have been having students create accounts at www.vimeo.com where they can upload short clips and send me a link which is password protected. I receive a link to these short videos a few times during the week. I usually check my email at night, so it takes me a few minutes to look at the video clips and email back a very short response. But we are accomplishing a lot more within a week doing this. The student is receiving my feedback a few times a week, rather than just once.
A typical example of what I might ask a student to record would be this: please videotape the 2nd page of Knight Ruppert, counting aloud with the metronome at 80 by Tuesday. My requests can be very specific, and I can encourage the student more frequently throughout the week.
A bonus: videotaping short clips from home also lets me see how the student sits, what his piano sound like, and if he can find his metronome! It is working especially well with middle and high school students, to keep them practicing and responsible for themselves throughout the week.