This depends heavily on the occasion.
Generally we recommend sending e.g. an invitation a 2nd time 4-5 days after the first send out - to the marked WHITE (delivered) group. These recipients have yet to open their invitation.
Depending on the importance of your event, messages can be sent to the groups WHITE and YELLOW to remind them to reply a couple of weeks before your event.
Many customers also send messages as event reminders to the GREEN group a couple of days before the event.