You can define the time for payment in billwerk when offering payment "On Account". The time for payment will be displayed on the invoice. Go to "Settings > Payment > Payment Settings > Black Label Providers > On Account" and click the "Edit" button.
Now you can select the wanted time for payment and press the "Save" button.
The time for payment will now be given out on every invoice for customers paying on account.
For automated payment On Account via a payment provider, the payment term is configured in the settings of the payment provider.