Depends on the charger but either effectively zero or considerably less.
People get pissy about it, but think of electricity like water. Having a longer pipe is a negligible amount of water if the faucet is still off. And the faucet can only turn on if your device completes the circuit by being plugged in (and doing the appropriate handshakes)
That said, some chargers will consume a negligible amount of electricity to actively listen for a device. Think of it like the water in your toilet. Every so often enough evaporates or leaks that you hear it run a bit to refill. But mostly it is nothing until you flush.