Engage an event marketing agency as soon as the event becomes too big for your internal team. Concrete signals:
As for timing: call an agency at least 12 weeks before the event. For large events 6 to 9 months in advance. At Live Impact we prefer to start right at the idea stage. The first weeks determine whether the concept becomes distinctive or generic. Engaging an agency later as a firefighter rarely works well. Plan an exploratory meeting.