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).
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.
Tyson teamed with DC Comics to track down a Krypton-like system that matches hints from the comics. He found a fitting red dwarf in the constellation Corvus (the crow) in the southern sky, a mere 27.1 light years from Earth.Tyson even gets a cameo in Action Comics #14, "Star Light, Star Bright," when Superman makes a visit to the astrophysicist's planetarium.