You can set a default time off duration for your employees that differs from the one you set in the time off settings.
To do this, first go to your employee's profile by clicking Scheduling > HR > Employees > select an employee. Once in your employee's profile:
Click the Settings tab.
Toggle Override the organization's default time off duration.
Enter the desired value.
Click Save.
This value will automatically be used when you accept a time off request or create a manual time off. Note that you can still modify it if necessary.