For a staff event for 50 to 200 people, expect 6 to 12 weeks of preparation time. The first weeks go into determining the format, date and venue choice. Then come programme development, catering, communication to staff and technical production.
For larger events (500+) or special venues, 16 weeks is a more realistic standard. Don't underestimate the internal communication: invitations, sign-ups and updates take more time than you think. An agency like Live Impact takes the full project management off your hands, so you can concentrate on your own role.