To add to what Flack said, make sure that whatever you choose to do for volunteering, you can talk with great interest and even passion about. During one of my interviews, we spent the entire time talking about my volunteer work since it was something that they could tell I was excited about. It had nothing to do with medicine, nor did any of my volunteering. Like some of the other people posting have said, if you volunteer in an ER or at a hospital, you are often doing scut work like transporting specimens and such--not that this is something bad--but I wanted something unique where I was not just another number. So, if being in the ER, no matter what you're doing, excites you and you can talk passionately about it, go for it! If not, I always suggest to find something else and to never volunteer at something just because you think it will look good on your transcript--interviewers see right through that kind of thing.
As for the number of hours, that's something that typically doesn't even make it to your CV (r?sum?), so just make sure you have quality and not necessarily quantity.
Yes, I am procrastinating because I just don't feel like studying. Only two more delightful days left of third year.
--Brendan--
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