Allow at least 3 to 6 months of preparation time. For larger groups (50+ people) or exotic destinations, 6 to 9 months is more realistic.
The timeline: month 1-2 for concept and destination, month 2-3 for bookings and logistics. Month 3-5 for filling in the programme and communication. The last weeks are for fine-tuning.
Bring in an event agency if you want to do it professionally. They know the destinations, have contacts and take the organising off your hands.