AirTag battery life seems to depend on where it's used

To be honest, I have yet to use an AirTag with a wallet, so I really can't offer much advice related to this. Sorry.

However, regardless of what you place it inside of, I would follow these "rules" for best placement:

• If you're placing an AirTag in, let's say, a leather wallet, ideally you want the white plastic side (the speaker side) facing outward, toward the leather. Here’s why:
  • Sound Performance: The plastic side is where the AirTag emits sound. If you're relying on the “Find My” app to play a sound and locate your wallet, having that side face outward gives the sound a clearer path to escape the wallet.
  • Signal Consideration (Ultra Wideband + Bluetooth): For signal purposes — both Bluetooth and Ultra Wideband — the AirTag is designed to transmit in all directions fairly evenly. That said, metal can slightly interfere with wireless signal transmission compared to plastic. So if the metal side is closer to your cards or anything else metallic in the wallet, you might reduce the effective signal slightly. Keeping the plastic side outward generally minimizes that issue.

In short, plastic side out, metal side in should be your best bet for both sound and signal clarity from inside a wallet.

As an option, they sell these slim AirTag holder inserts (like those wallet card-style ones).

The "why" is because the tag will have to worker "harder" to broadcast its Bluetooth signal through your wallet, then when it would be attached outside of it. By doing so, it will drain its battery faster.

FWIW, I don't use them inside of anything. Instead, mine are all attached to my various vehicles' key rings. Using them as such, their batteries typically last 9-12 months for me.