for non-trad, maybe a four hour shift twice a month (think 1/2 day or an evening every other weekend) for a year or two while you work on pre-reqs and it should be in a clinical setting if your paid work isn't clinical.
For a college student, ideally at least 2 years of 2 hrs/wk with increasing responsibility in the organization or by directing/training new volunteers.
Please note that the duration in months as well as the hours is a measure of your dedication to community service.