(There's a shadowing poll of accepted students.....some had over 400 hours, some had none. If you do a search, there are some good threads.)
But, short answer: 60-80 is a good minimum. Do what you can/what doctors will allow....some expect you to shadow for a day all day, some expect a few hours/week for a month. It's better to get somewhat of a variety....I've also heard that shadowing primary care is something adcoms like. I know it's been helpful for me. Also, start as early as you can...that way, you can get in 100 hours spread over 3 years (much easier to do) than 50 hours in the two months before app season!
Also, keep track of your shadowing - hours, hospital, etc.
..... And don't do it because you feel you "have" to - do it because it's fun, you get to see what it's really like, you get to see fascinating social interactions, and it has the opportunity to really change your perspective on who doctors are!
