It depends on a lot of factors, but it can take anywhere between a day and a few months, but normally within the course of a week or so. They will grieve her loss for a few days, then they will either simply move up one spot in the pecking order or rearrange the whole thing, the latter taking...
Im sorry for your loss.
Normally the second in command will become the head, though sometimes the whole pecking order will reset in a different order and a random bird will become the head.