For members, the Anytime Fitness app will prompt you 75 days after you were first issued a free subscription to redeem your next free 3 month subscription, or cancel your subscription.
You can renew your next 3 months via the AF App.
Just like with other subscriptions on Apple devices, you can check when the offer ends by going to Settings > [your name](Apple ID, iCloud, iTunes & App Store) > Subscriptions.
For non-Anytime Fitness members, the paid subscription will automatically renew after 3 months from redeeming the code unless you cancel at least 24 hours before trial period ends.