What can you fit into your schedule that won't overwork you? I assume you're taking some difficult classes, perhaps studying for the MCAT, maybe a job on the side for some cashflow, and don't forget free time to unwind.
Most schools I've looked at don't have a minimum requirement of clinical hours. The important thing is that you at least have enough hours to prove to them that you've been exposed to the world of healthcare enough to know that this is something that you are comfortable doing for the rest of your life. If you think you can do that with 100 hours, fine. If you think you need 1000 hours, so be it, but generally, more is better.