Clinical experience, whether in the form of shadowing or volunteering or work experience is a definite plus in the admissions game. I think it's the 3rd most important item behind gpa and MCAT, but this also means it shouldn't be done at the expense of these two. Any amount of time will do, although putting in enough time to get a letter of rec out of it would be ideal. Plus some people get in there and realize they don't want to help people quite as much as they thought they did.