In the Strait of Georgia/Puget Sound region a GPO's normal lifespan is about three years. There is a school,of thought that a male GPO that doesn't breed may live as much as a couple of extra years. OTOH: experts such as Jim Cosgrove of the Royal British Columbia Museum tell me that there is no concrete eveidence that this is so.
It's also possible that GPO's from Alaska or the Sea of Japan might live somewhat longer than B.C./Washington animals due to lower temps but I don't think that's been confirmed either.
It's pretty common for public aquariums to change out their GPO's every year. The reason is that they really aren't all that "giant" until the last year of their lives. Further, when they do reach an impressive size it's just about time to mate and that's best accomplished back in the natural habitat. Some aquariums do breed them in captivity from time to time (the Seattle Aquarium for one) but I suspect that they don't devote public display space to a brooding female.
Visitor: Where's the octopus?
Docent: Well, you can't see her because she's been hidden in her den for the last five months.
Sorry, they need a very large tank - like 800 gallons. Also, GPOs come from a cold water area and need a chiller. And if you think your little octopus eats a lot, just imagine the sizeable crabs a GPO has for dinner.
It all depends at the temperature at which you keep them. Lab studies have shown that you can significantly increase life span of octopus when you keep them at lower temperatures. Also a 2-3 degrees decrease in temp can increase 15-20 days the embryonic dvmpnt (for Sepietta oweniana) and result in larger hatchlings. This varies btwn geographical regions.
I don't believe captivity alone can be accounted to increase lifespan. In captivity, feeding rates may be increased, which might have an effect on size and time at maturity but will not increase lifespan rather the opposite.
For cephs, it all depends on temperature!!