Well, the lifespan for the dwarf octo itself might be 6 months to a year, but there's no way to buy one to be sure of its age--you might have a couple of weeks or a couple months, but likely not more than that.
Even the bigger species that most of the 'serious' keepers have (bimacs, briareus, etc) don't live longer than a year, and usually considerably less.
Dan