How long was the longest solar flare measured?

Answer provided by Mitzi Adams:
”I’m not really sure how long the longest flare might have been.  That is a difficult question to answer, since it depends on how you are measuring the flare.  I can tell you that a flare that happened 21 Mar, 2011  (a C-class flare) lasted about fifteen hours.  That is, the X-ray flux from the flare in one energy band took that long to return to its pre-flare level, as measured by an instrument in orbit around Earth on the GOES satellite.  The first X-class flare of solar cycle 24 lasted only about an hour, however.  If you would like to see the graphs yourself, go here:
http://www.swpc.noaa.gov/rt_plots/xray_5m.html

For information about flare classes, visit this website:
http://spaceweather.com/glossary/flareclasses.html‘
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