The best way to do this is by Daily sync with the external system sending an event to Appian in case of any data change. You can configure a web API that will receive the record identifiers which are newly created or recently updated by the external system. This way you will never go out of sync.
If the event mechanism is not possible then go for Daily sync. But in this methodology sometimes record data might not be live/correct, so whenever you are displaying a summary record/ consuming in a related action do the live calls to avoid any kind of discrepancies.