The data rate depends on if it is using the 2.4 GHz band or the 5 GHz band, both are possible with n-protocol unless you have limited your wireless to only one band. The data rate in 5 GHz is double that in 2.4 GHz. Also, you have to know what the protocol of the network is, the AirPort card can only transmit at the rate of the network, so if you are on a g-protocol router, that is the fastest rate you can see.
And then interference, attenuation, etc. will drop that rate.