Ziegler provides a cool intro, implementation sample and much more. When deployed we can apply this policy to a contenttype in the UI, or in code. To create our own expiration logic we have to implement IExpirationFormula and its ComputeExpireDate:

public class TaskExpiration : IExpirationFormula
{
	public DateTime? ComputeExpireDate(SPListItem item,
						XmlNode parametersData)
	{
		if (!item\["Status"\].Equals("Completed"))
		{
			return null;
		}
		var dt = (DateTime) item\["Modified"\];
		return dt.AddDays(30);
	}
}

In order to see IExpirationFormula, add a reference to Microsoft.Office.Policy (and maybe Microsoft.Office.DocumentManagement): To see our custom retention policy, we have to register it in xml, we can do it in Feature Receiver like Yaroslav: