I've never known there to be a certain amount of walking to help but whatever you can handle without wearing yourself out too much. If you have BH already then walking will deffinetly help no matter how much you walk. If you haven't started having BH then I'm not sure that walking will help start those.
@itscatt, I need to try and jump start my labor cause my doc will be out of town when I have my c section.
Walking will help the baby settle lower into your pelvis.