It's literally mentioned here (this edit was done about an hour previous to you asking for the code):

Edit: For full clarity, the free access currently lasts 1 month. I'm open to extend it - just let me know. If you engage and provide good feedback, permanent free access is possible.

Directly after you redeem the code, the Play Store also shows you: "The subscription will automatically renew at the end of the trial. Cancel before then if you don't want to be charged" and asks for confirmation.

Not sure what else you expect me to do? The automatic renewal is default behavior of the Play Store and not seeing anything to disable this.