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 > click employee you wish to make changes for
Once in your employee's profile;
Click the Settings tab
Activate the Override the organization's default time off duration toggle
Enter the desired value
Click Save
This value will then be automatically used when you accept a time off request or create a manual time off. Note that you can still modify it if necessary.