When sending invoices to your clients, most of them have the same invoice terms (due date interval), which can be set by clicking on Your name (bottom left) → Company settingsProject accountingInvoicingDefault invoice terms and set the desired number of days when the invoice will become due.

If some clients have a different due date interval, you can go to Clients → Select the desired client → Edit Client → Default invoice terms and change the due date interval from there. 

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