Is it ever specified when did Krypton explode considering Earth time?
I read that Superman took about three years to reach Earth, but I think it was considering the time at the capsule (which would change based on its speed).
In the DCnU reboot:
27.1 years ago.
DC Comics teamed with Neil DeGrasse Tyson to find a star that would resemble Rao. In Action Comics #14, the light from Krypton's explosion was supposed to be visible from Earth. This news is related courtesy of CNET on November 6, 2012.