It depends on the requriements and the size of your database start with daily sync and change the frequency if needed . few best practices Avoid slow and costly initial sync, If possible, start with data in only one of the sync group's databases. A sync loop occurs when there are circular references within a sync group. In that scenario, each change in one database is endlessly and circularly replicated through the databases in the sync group. Ensure that you avoid sync loops, because they cause performance degradation and might significantly increase costs. Avoid out-of-date databases Also ref this article for more details on sync best practices
https://docs.microsoft.com/en-us/azure/azure-sql/database/sql-data-sync-best-practices
