I believe you can use it whenever YOU feel comfortable using it. Although, you certainly wouldn't call your boss at work or your teacher 형/누나. When it's a work place, I think it would be safe to stick with 선배님/선생님/사장님 or other terms. If it's a casual encounter, you may start those terms immediately if you would like.

If you are still at school, people won't feel awkward to call like that even though it is first met. However, after graduating university and you get older, it gets awkward to call someone using that name. in that case, as Richard already replied, you can call them like that when you and they both feel comfortable and close enough.