Computer fan tack signal (4 wire)

[sid3]

Hello,

Question: 
The documentation from this site states that the tach wire is pulled to ground for every revolution of the fan.  How long is it pulled to ground ?  the entire revolution until the next ?  x number of milliseconds ?
I don't have a setup right now were I can measure it.  But I wanted to know if someone had some documentation on what its supposed to be.

Any help is appreciated.
Thanks
Sid.