how long do I have my apple tv subscription for?

Any existing subscription (with Apple) will auto-renew each period until canceled.

See or cancel your subscriptions - Apple Support

The regular free trial period for the Apple TV+ subscription is 7 days, exactly as indicated when you subscribed. You can look up the last day of the free period in your subscription overview.

(To qualify for the free year, users should start the subscription within 3 months.)

You can pay monthly or yearly, you can cancel when you want and your service will end at the end of the then current subscription period