Partly it depends on the type of medical school your child is most interested in. Some programs are more service oriented than others.
Shadowing 40-60 hours, with at least a quarter to half of the hours with primary care physicians. More than 75 hours is overkill.
Community service w/ disadvantaged/disparaged groups-- aim for 150+ hours. Longitudinal service with a single organization or cause is strongly preferred over multiple short term, one-off projects. Service should be of the hands-on, in-person variety, not fund-raising. Service with the Peace Corps, Teach for America, Americorps is looked upon extremely favorably by med school adcomms. So is military service.
Clinical exposure-- aim for 150+ hours.
I would say these are probably the bare minimums one should have to have any reasonable expectation of getting a med school admission. Many applicants will have more, some will have significantly more. Working in a clinical exposure job during gap year(s) is very common.
However, what is more important than the absolute number of hours is what a student takes away from those experiences. Community service and clinical exposure are meant to be learning experiences for the student. A hundred hours of a meaningful activity where a student gains deeper insights about themselves, about medicine and about the human condition is better than a thousand hours of check-boxing busywork.