My windsock is erect, what does this mean?
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.)