Your italki Credits will not expire as long as you are an active member of italki.
italki Credits will be valid for one year after your most recent login date. If you do not log in at least one time in one year, then your italki Credits will expire and be deleted from your account.
This is done to maintain updated and accurate records.
All you need to do is log in at least once per year to ensure your italki Credits remain valid and available for you.