The default "Finish Time" parameter is automatic and cannot be modified.
A possible workaround would be to create a custom field.
1. Open the "Task" tab
2. Select Task | Advanced | Customize
3. Add a new field called ‘Finish Time’
4. Select a Date/Time data type
5. Set the default value
6. Click "Save"
Open a Task and select the "Custom" tab, this will display the "Finish Time" field.
A caveat to the above is you will still have the automatic value that occurs when you close the task.