How far can the Air tag be detected?

Directly the a Bluetooth signal is good to about 100 feet. Maybe double that if conditions are ideal. Could also be less if you're in a building.

Don't forget that Airtags can take advantage of being detected by devices other people have set to use Find My Network, so as long as somebody else is nearby with that active it can be pretty much anywhere.

Set up Find My on your iPhone, Mac, AirPods, or other devices - https://support.apple.com/HT210400#enablefindmy

"To see your device even when it's offline, turn on Find My network. The Find My network is an encrypted, anonymous network of millions of Apple devices that can help you locate your device."