The what's new summary emails are sent either immediately, daily, weekly, monthly or never depending on workspace notification settings and personal notification settings.
If settings are set to immediately, the email are sent right away when there is a new notification in the workspace and the user is not online. The immediate email will be sent once in a 12h period, to avoid spamming. If the user visits the workspace the 12h limit resets.
If the setting is set to 'never', no email is sent even though there might be new notifications in the workspace.
Daily emails are sent every day during the night in Europe.
Weekly emails are sent on Fridays at 4 am EET.