My windsock is erect, what does this mean? I know that airport windsocks are calibrated to reach full erection at a particular wind velocity. At half that speed, the first half of the windsock should be erect and the second half should droop sorrowfully. At one third the speed it will be one third, and so on. 

Trouble is that not all windsocks are calibrated the same way. How do I find out what wind velocity will bring my airport windsock to a fully erect state?
Windsocks come in various sizes and speeds, but an FAA Standard Wind Sock aligns with the wind at 3 knots, and is fully extended at 15 knots like CGCampbell pointed out.

A typical wind sock (at least around here) looks something like this:


The stripes are actually useful:  


The first stripe indicates a 3-knot breeze
(The wind sock has turned and aligned with the wind - usually you should look for an anemometer near the wind sock to determine if there's any appreciable wind because often the first segment is held open by the frame.)
The second stripe is ~ 6 knots
The third stripe is ~ 9 knots
The fourth stripe is ~ 12 knots
The last stripe is 15 knots or higher
If the windsock is missing you probably shouldn't be at the airport.
If the pole is missing you're probably on your way to Oz.


(You can actually get a far more precise estimation if you're willing to do some trigonometry, but this works well enough for most purposes.)