Mr Light wrote:

Most comic books only take place over 1-2 days. And each comic ends on a cliffhanger that leads right into the next. So 700 issues of Superman would only equal 1,400 days, or 3.8 years.
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And then you count time between story lines. Normally five years of real time is considered one year of comic time. Marvel seems to reference this every once in a while, and there was supposed to have only been a little over two years between Superman's death and Infinite Crisis I think.