With the right input, you can arrive at a good kick-off theme in one session. A clear annual message and an hour of brainstorming are enough. Without that basis, you go round in circles for weeks.
The mistake most organisations make: coming up with the theme before the strategy is clear. Start with the message, not the creative name. Plan at least 6 to 8 weeks before the kick-off to properly develop the theme, invitation and programme.