Expiry date is the date that an item listing will expire. If you do not provide this attribute, items will expire and no longer appear in Google Shopping results after 30 days. You cannot use this attribute to extend the expiration period to longer than 30 days. Collect this from google documentation. https://support.google.com/merchants/answer/188494?hl=en
Others solution, create feed file automatic fetch schedule from your merchant account. Hope your problem will solve.