SmartMail's Smart Segments automatically update :
Every 30 minutes if they are connected to an active automated flow
Every 24 hours if they aren't connected to any active automated flow
On top of that they are updated again whenever an email to the segment is sent. Segments are set to auto-update at 4:00am UTC.
If you'd like to manually update your segment before sending a campaign, click the refresh button while on the Smart Segments tab.
Note: Most segments will update within a minute, but larger lists can take up to 5 mins to populate.