We cannot rell how much time it wil take to sync. But it is not the number or rows that matters most, but rather what operations (INSERTS, UPDATES, DELETES) will need to be performed (if there is nothing to do it will take a few minutes only). So the first sync may take considerable time, but sync job following after may take much less time. So the general advice would be that sync job should be scheduled so frequently that sync time is acceptable.