Unless you have very high new volunteer volume, I'd personally recommend once or twice a month to get more folks in the orientations and to maximize the time your staff or experienced volunteers spend training new people. We've been doing weekly orientations for a long time but sometimes only have 1-2 new people per week, which is inefficient and sometimes awkward, and sometimes no one new at all which wastes our time waiting around. On the other hand, we sometimes have ~10+ people, and more than that can get unruly.