Syncing happens a few different ways within BankToSheets.
The first sync happens when you connect your institution to the Sheet. BTS will try to search back as far as available (within Plaid). This will be 2 years, or 500 transactions. This can vary greatly though from bank to bank. Some banks will only pull in a couple months of transactions.
Once your institution is connected to the Sheet, then the Sheet will update twice a day. It will sync between 12-1am EST and 12-1pm EST. The best way to know when it syncs is to look at the Balances tab. It will show a timestamp of when the sync occurred. Since the sheet should already contain as far back as possible at this point, this daily sync only looks a few days back to make sure no transactions are missed, and it will only add the transactions that are currently not in the Sheet.
The last way to sync is with the Force Sync button in the menu. This will also only sync back a couple of weeks. The main reason for using this feature is that it will trigger the Auto Categorization. If you've already synced your transactions to the Budget Template, and then you create your budget categories, then you'll want the new categories applied to the transactions. In order to do this, you'll need to run the Force Sync.