Once they have enough applicants approved? There is a backlog of approved applicants waiting for oath, or at least that's what it sounds like from one of the responses someone else got after emailing Niagara Falls. So probably there is a shortage of staff, including people taking summer vacations over the past few months?
But if you need to travel for work soon, you should write to them asking to be scheduled for oath ASAP. They might also ask you for a letter from your employer explaining your need to travel soon.