Currently, the expiration date (ID: duedate) on Estimate record follows what is configured on Setup > Sales Preferences > General > Default Estimate Expiration (in days).
1. Navigate to Customization > Scripting > Workflows > New.
2. Set the following:
- Name: (Enter a Name for your Workflow For example, "Estimate Set Default Expiration Date"
- Record Type: Transaction
- Sub Types: Estimate (Quote)
- Execute as Admin: is marked
- Release Status: Released
- Event Based: Marked
- On Create: Marked
- Trigger Type: All
3. Click Save.
4. Click New State button on the left pane.
5. Enter a Name (e.g. Set Default Date) for the new Workflow State then click Save.
6. On the left pane, click the New State created in Step 5 (e.g. Set Default Date).
7. Under Actions tab, click on New Action button.
8. Choose Set Field Value for the Type.
9. Trigger On = Entry.
10. In the Parameters section, select Due Date on the Field dropdown.
11. In the Value section, mark Date and select END OF THIS FISCAL YEAR in the dropdown field.
12. Click Save.
The Date on the Expires field on Estimate should show 12/31/2014 and would override the Sales Preferences Default Estimate Expiration (in days)