How long is a "moment of silence"?

When people observe a brief collective silence, e.g. in memory of victims of war or terrorism, the usual period is two minutes. See, for example:

http://www.momentofsilence.org/.

http://www.guardian.co.uk/Guardian/gallery/image/0,8543,-10404263345,00.html.

As long or as short as you want it to be, this usually depends on the circumstances and the people involved.
FYI, it took almost two and a half hours to read all the names of the people killed during the attack of the World Trades Centre buildings. There were 2801 of them.
http://www.cnn.com/

At league clubs around Australia the remembrance silence has become part of the nightly nine o'clock ritual, when any light other than a memorial flame is dimmed, members stand in silence, and then recite the Ode.

A brief silence, usually one or two minutes, characterises many other remembrance ceremonies throughout the British Commonwealth.

The concept of a remembrance silence appears to have originated with an Australian journalist, Edward George Honey...

...Honey published a letter in the London Evening News on 8 May 1919 under the pen name of Warren Foster, in which he appealed for five-minute silence amid all the joy making planned to celebrate the first anniversary of the end of the War.

...Sir Percy wrote, 'When we are gone it may help bring home to those who will come after us, the meaning, the nobility and the unselfishness of the great sacrifice by which their freedom was assured' https://m.reddit.com/r/Military/comments/2lvv6t/remembrance_day/
The Ode:
They shall grow not old,as we who are left grow old.
Age shall not weary them nor the years condemn.
At the going down of the sun, and in the morning,
We will remember them.

One minute is the most common but others have been longer.
https://en.wikipedia.org/wiki/Moment_of_silence

One minute is a common length of time for the commemoration, though other periods of time may be chosen, normally connected in some way with the event being commemorated (there might be a minute given for every death commemorated, for example). During the moment of silence, participants may typically bow their heads, remove hats, and refrain from speaking or moving places for the duration. A person officiating or presiding over the gathering will be responsible for the declaring and timing of the period of silence.

[Added text - McG]