AirDrop works outside of whatever WiFi networks are around and doesn't need a base station. It is like a mesh network where all the devices just talk between each other. If nothing else is "sharing" the road - you get the full speed. The further away you are or the more interference, the slower it gets. AirDrop will work by passing the data from Mac to Mac so it will get the file there (although slower) if it has to jump once or twice along the way.
That being said, for two close machines, the transfer speed should be the max wireless speed between your two machines. Hard drives are almost always faster reading and writing than WiFi speeds, so the radio link is generally the slowest.
However, all of that is just theoretical. You can check your actual speed by opening Applications > Utilities > Activity Monitor
Click the Network tab on the bottom and you should see stats about how fast the data is being sent and received. Detach from WiFi base stations to let AirDrop get all the hardware if you want the fastest transfers - leave it on if you need internet while dropping that large file.