The admissions committee is interesting in learning whether you have actually experienced the life-style of the medical profession. Because of that, there isn't really any set number of hours that the admissions committee is looking for. How could you compare 200 hours of folding paper "puke trays" by yourself in the ER waiting room versus volunteering 20 hours in the hospital nursery talking to the nurses, doctors, and techs? Which experience would you rather have?
With that said, most everyone who applies has volunteer experience of one sort, or another. I'd say you'd probably want 50-100 hours of volunteering at a single site in order to keep up to everyone else. I'd look for a volunteer job that allowed you to interact with patients, and health care workers. You could look to local clinics, or perhaps the neighborhood hospital. Don't be afraid to ask around. It's worth it if you land yourself an interesting job; the hours just seem to fly by and it doesn't feel like you're working. I believe that will give you the most benefit, and enjoyment in your volunteer experience.