It depends how your local DO processes these requests, how often the ceremonies are held and if everything on your N-400 is complete. So you may get just a few days notice or few months.
In Minneapolis-St. Paul, I got my letter 3 days after the interview.
I was asked to appear at the oath ceremony after 6 days. The interview officer told me that the ceremonies are held every month. And at the end of the interview he said that everything looks ok so I should be getting the letter within a week to attend the next oath ceremony.
Hope this info helps & good luck.
