How big is your shop? How many reliable volunteers do you have? Can you keep enough goods stocked on a daily basis? These will all play a major factor in your business hours. If you have a great little shop, people will make a point to get there during your hours. Our town has a small shop run by the "Pink Ladies," benefiting our hospital. All personnel are volunteers. They are able to keep items stocked because of the limited hours. (W-Sat, 9:30 to 3:30)
I can't say this enough to new owners; keeping your shelves full is very difficult.
When we first opened Big Thrift, we had our goods displayed on shelves throughout the store, and each shelf had items on it. After the first day, half our inventory was gone, and I had no idea where to get more for the rest of the week. (At the time, we were going to garage sales for goods - terrible idea.) Day 2 was a disaster! You only get one chance to make a first impression, and the second group of people were not impressed.
Be realistic on what your volunteers are committed to, and how much merchandise you can bring in each week. Little shops can sometimes do better if they can keep their shelves full, and create some anticipation for the buyers.