Depends on the type of power transformer in there. The latest generation of "Switched-mode power supplies" is quite efficient and does not draw that much when not in use, compared to traditional ones.
And there are two reasons for making it very hard to actually measure the power it draws with a regular electricity meter. First, it's just a very small amount they consume, and, second, it uses pulse-width modulation techniques with adjustable duty-cycle on the AC side
While newer generation chargers/AC-DC adapters will consume just tiny bits, it is very advisable to just remove them from any power source for power saving reasons. I use power strips with a switch to cut the power completely with ease when I'm gone or asleep.