You can set the payment terms for billing in the following way:
1. Open the Terms screen from the View menu.
2. To create a new payment term, click New.
3. Enter the Term Name (say 21 Days) and number of Grace Days allowed, say 21.
4. On the Project-Billing tab, you can select the desired Payment Term for the selected project.
5. Save the information. You can select a term in the Client screen as well to apply to all the projects of the selected client.
