An iridium flare happens when communication satellites become positioned so that the Sun strikes the metallic antennas and thus reflected to the Earth.  These reflections can be very intense.  In lots of occasions the flash is so strong that it can reach magnitude -9, which represents 30 times the bright of the Venus planet. The flashes can be very easily seen and the only requirement is that the observer knows where to look and at which moment. The duration time is not fixed, but we already witnessed flashes of up to 15 seconds.

There are many websites that track these satellites and can give you exact timing of them from your location.  Heavens-above.com is one and there are more.  There are apps for your smart phone as well.

 

Here is a shot from The Nebraska Star Party where we had 2 satellites about one minute apart.  Unfortunately the tree got in the way for some of the flare.

 

IMG_9430

 

 

 

If you would like more information on iridium flares http://www.astrosat.net/us/flare.php has a great write up.