The current offer is for 3 months and it must be claimed within 3 months of setting up the new device; it is also limited to one offer per family. See: https://www.apple.com/promo/pdf/EN_US_ATV+_Promo_TandCs.pdf
To claim, see the following from https://offers.appletvapp.apple/tvp01213mf
You can start your 3 months free subscription to Apple TV+ after setting up your new Apple product. Here's how to redeem your offer:
Turn on your new Apple device and sign in with your Apple ID.
Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.
The offer should be presented immediately after launching the app. If not, go to the Apple TV+ tab where you'll see the offer.
Tap Enjoy 3 Months Free.
Check Eligibility