Setting expiration date in a content type policy
1. This will work if you have SharePoint Standard or Enterprise edition. Open your site in a browser.
2. Select "Site Actions > Site Settings".
3. Click "Go to top level site settings". Of course, you don't see this link if you are already on the top level site.
4. Click "Site content types" under "Galleries".
5. Locate the content type that you want to use for setting the policy on. Click the link of the content type.
6. Click "Information management policy settings" under "Settings".

Fig 1: Settings

7. Select "Define a policy..." option and click OK.

Fig 2: Define a policy

8. Check "Enable Expiration" checkbox. Select "Created" or "Modified" in the item's properties and select days, months or years from the last drop down in the row. Enter a number in the middle text box. Now select action. You can choose the default action "Delete" or you can start a workflow and do custom actions. Click OK to save the settings.

Fig 3: Setting Expiration

Hope this will help someone. It is straight forward but I have seen users asking this question "How to set expiration policy on a content type".