i think the best advice you could get on volunteer work/quanity would be to take your application to ADCOMs of local schools/school you are applying to. I know all the schools I was interested in were more than willing to sit down and review my application before i applied. they set me straight on where i stood and what i could do to benefit my application.
in my opinion though, it really depends on a lot of factors.
first is the school you are applying to. the med schools i applied to viewed different experiences under different opinions. for example,i did 2000h of sports medicine for my school's football team, the med schools heavy in primary care didn't really care/consider it medical experience and suggested to volunteer more at a hospital (i had done 4/wk from 16-21). however, schools that turn out more specialists could see sports medicine it as an interest/experience related to ortho.
second, your interview/application allows you to shed rhyme and reason for your volunteer efforts. therefore, if have other volunteer interests than being in a hospital, do them. i volunteered in hospitals and i shadowed physicians but i prefered to be involved with food banks and helping lower income families because i felt it was more benefical and interactive. therefore, i used my application/interviews to show that i was dedicated to helping people, but that i felt experiences other than hosptial volunteering (ie making coffee and giving hospital directions) were more benefically to me and the community.
but you do need significant hospital experience/exposure for your application to be able to show you know (or at least you think you know) what the medical environment is like. also to show certainity that medicine is what you want to do...
sorry a very open ended answer, but i hope it helps some....
