Adcomms like to see nonmedical volunteering too. It shows you are involved in the community and care about people. I agree it is good to have a volunteer experience that lasts 1.5-2.0 year or more, but a lot of applicants have short community activities too. It's fine to list all these in one space, naming the space Short-Term Community Service. In the narrative you can list dates, name of organization, total hours, whether it be 1-8 or 40, and what you did. It doesn't need documentation.
So you could have something like: helped at a health fair handing out brochures on good diet=4 hours. Assisted new students on move-in day=6 hours. Usher at charity concertX4 hours for three events=12 hours. Ran in race for the Cure, raised $200=3 hours. Alternative spring break, built homes in S. Texas for migrant workers=40 hours. Taught religious education 1 hour/week for 40 weeks=40 hours. And so on. Give a grand total at the end. Obviously you wouldn't do this at all if the total is 2 hours. But over three years these things can build up into a nice grand total.