When they say someone ruled for 800 years, what did they think they were talking about? What was their justification for thinking someone could live that long? Were there just 20 monarchs in a row who used the same name? Is it a hyperbole created by the fact that monarchs would have been healthier and lived longer? Were they merely filling in the gaps for periods where they didn't know who the ruler was?
/u/kookingpot and /u/husky54 addressed the SKL in this thread about the fantastically long-lived biblical patriarchs. (See kookingpot's answer here and husky54's response here.) I tend to agree with the latter that the reigns were methods of "stretching" history and the names of recorded kings far back into prehistory; people in Mesopotamia knew their origins were ancient, but they were rather off the mark about exactly how ancient Mesopotamia was.
To their responses, I'll add that assigning fantastically long reigns to early rulers is known from other cultures as well. In Egypt, for example, Manetho recorded that the first dynasty of (divine) kings ruled over Egypt for 13,900 years. See Berossos and Manetho, Introduced and Translated: Native Traditions in Ancient Mesopotamia and Egypt (pp. 130-131).