I think that the further in the past you go, the more nebulous the time frame and continuity. I think they said that the time since "Siege" was one year, and that "Dark Reign" lasted a year... but I'm sure the events of the 70s only took place over a single year or so.
If you think about it, one comic book usually takes place over one day. And the end directly leads into the next issue. So if one issue equals 1-2 days, then a 700 issue comic book is only 1,000 days or so.