I get the impression that it seems to matter very little, whether you've thrown in thousands of hours into volunteering at the very top schools. Why? Because those thousands of hours were low-impact activities, and the top schools look for high-impact activities. Reality doesn't judge performance in terms of time taken, but rather outcome. If I'm mistaken, feel free to correct me. This is just my gut feeling. I just get the sense that, while one should have a few dozen to a hundred hours for the sake of getting a sense of the hospital, unless one's hospital volunteering very unlikely happens to be a high-impact activity, it would be better to invest that time (if you're doing more than your dutiful 3-4 hours a week) in other higher impact extracurriculars (assuming enjoyment is equal).