Hi @Josua Köhler,
AFAIK there is no graph API to fetch the license expiry date, if you want you can check in admin portal.
But you can raise a feature request for the same and upvote it so that it can be considered and taken in consideration: https://techcommunity.microsoft.com/t5/microsoft-365-developer-platform/idb-p/Microsoft365DeveloperPlatform
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".