The species we most commonly see have a lifespan of about a year, the bimacs seeming to live 18 months if kept cold (Roy had one that lived almost 3 years but that is the only record we have of any octopus living that long). Since most that we keep in an aquarium are wild caught and vary in age, we can only make conjecture about how much time we will have with them. I did not release either of my mercatoris broods to the main tank until they were 5 months old and they were still very recluse for about a month. Using this as a go by, observing the one species that seems to be somewhat age identifiable by mantle size (O. briareus) and recording the reported deaths, six to eight months would be my expectation for a young octo and 3 max months for an adult.