Credits purchased from OpenAI expire after 1 year.
See the terms:
https://help.openai.com/en/articles/8264778-what-is-prepaid-billing
specifically: 4. Expiry and Refunds: Purchased credits expire after 1 year and are non-refundable.
Also please search before posting, as this question is easily retrievable upon searching in the forum as it has been answered before in the past.