You can set the default due-date for your invoices from the following steps:

1. Go to Companies option.
2. Select the Company for which you want to set the default due-date.
3. Click on the Edit icon from the top-right corner.
4. The Edit Company window will pop-up in the center
5. Scroll down at the bottom of the window and you'll be able to check the Default Due-Date option in the Settings Menu.
6. Select the default due date and click on Save.