Due date of a process is called process deadline.
Ways to set process deadline:
1. You can configure process deadline as a formula on template edit page.
It is possible to set process deadline as X calendar days/business days/weeks/months after process start.
Process start date + Number of calendar days/business days/weeks/months = Process deadline
Remember that once you start process from this template, you won't be able to change process deadline formula for started process. Then you can only set the exact date manually or delete it.
2. You can always overwrite process deadline with exact calendar date in the active process.
When you start new process from template, fill in the form with process name and deadline. You can choose exact date in the calendar or use buttons "Today", "Tomorrow", "In 2 days" etc. to calculate date for process deadline.
Please note that process deadline may affect process tasks' due dates if these are based on a process deadline (e.g. "5 days before process deadline").