http://www.washingtonpost.com/wp-srv/washtech/features/iridiumqa.htm
Q. What do the flares look like? How long do they last?
A.The flares only last for a few (10 to 30) seconds, and the maximum
intensity period is even shorter. They are point sources of light and move
slowly but perceptibly across the sky. The brightest flares can reach
magnitude -8, which is about 50 times brighter than Venus (Mag. -4), the
brightest star-like object in the sky. The flare rises gradually in
intensity to its maximum, then fades until it is no longer visible to the
naked eye (the satellites themselves are only magnitude +6 under normal
lighting conditions, and so can only be seen with the aid of binoculars).
Although the flare is not as intense as the full Moon (Mag. -12), they can
often seem to outshine it because they are point, rather than extended
sources.