Thanks for joining TONMO! As @OctoNation noted in his early reply on FB , factors such as water quality and enrichment can certainly have effects, so it's hard to know. If you are asking about tank-kept, in otherwise (or seemingly) healthy conditions, there are MANY examples of cephs that were kept for only 6 months to a year, but no particular species stand out as being less robust than others.
In situ, I believe one animal has been observed in the same location over a period of at least 5 years... someone may be able to correct me on that. But that's not your question
Shortest span is tough because there's so much we still don't know about these animals and tank-kept cephs have SO many variables.
I'm not sure that information is available. Generally speaking, the smaller the animal, the shorter the lifespan so you are very likely looking for a dwarf species, something the size of Octopus micropyrsus (mantle about an inch and arms about 3,5 inches). There are likely smaller in Japanese waters.