Not that I am a person to leave Google Play credit just sitting in my account, but in case you happen to be, take note that credit gained from the Google Play Opinion Rewards app will indeed expire after one year from the date it was credited to your account.
Here is what Google details on its support page concerning credit expiration.
Do my credits expire?
Yes. Google Play credits received expire one year from date of credit per the Google Opinion Rewards.
How can I tell when my credits will expire?
Google Opinion Rewards recognizes that it may be difficult to know when your Google Play credits will expire. We are currently making updates to give you notice of impending expiration.
If you like to carry an amount of credit in your account from Opinion Rewards, maybe it’s best to just spend it instead of losing it. That is, until Google provides a way for users to be notified of impending expiration.