I don't believe there is a minimum amount of hours of animal experience for any school. However, many schools DO have a suggested/minimum amount of veterinary experience hours. Veterinary experiences and animal experiences are listed as separate on the application. You will have to check out the websites of the schools you are interested in to get an idea of the number of hours expected.
Ohio State requires a minimum of 80 veterinary hours and Western U requires 500 animal hours. Those are 2 I know off the top of my head. I would look at the schools you are interested in to see how many hours are required, if any. Paying jobs do count as well as volunteer hours...but again double check with the schools you are applying to.
NCSU requires a minimum of 400 hours of veterinary experience, which can include appropriate research. At least 3 areas strongly recommended. Must be under supervision of DVM or PhD scientist in the case of research.
I think you need enough time to see the interesting and boring/stressful/difficult aspects of the different areas.