I don't know exactly, but I believe (anyone correct me if I am wrong) that they can stay out of water as long as it's gills are wet. Nancy has actually done a controlled out of water excursion with her bimac if I remember correctly..
Jean could give a better reply to this, given the excursions and escapes in the aquarium where she works.
It's my understanding that the larger the octopus, the longer they can stay out. It's not long, though. I never let Ollie stay out for more than a few minutes. She had things in mind to do, like feel all around the outside of her tank. Bimacs are sometimes found in tidepools, and they can go from tidepool to tidepool over the rock or beach, so they seem to tolerate short periods out of water. The danger is drying out. There are other Tonmo members who could tell you stories about octopuses out of water.
Mine is not a happy ending. My favorite octo was entering senescence but not so far along that I would have said he was close to death. He would wonder aimlessly sometimes and had lost much of his appetite but was still likely to have lived another month. I was changing out water twice a week because he had outgrown his tank (another story). During one of the extra water changes, I failed to close the lid (the way the tank was set up you could not tell the lid was open without looking directly over the tank, I had family coming over and was hurrying and the kids came in a little early). We are not sure how long he was out when we discovered him but he was not dry. However, there is an overhead fan in the room and his mantle had white spots. If he had been younger he might have recovered but he started eating his arms that night. He lived most of the week, eating more of his arms each day and I will not likely outlive the remorse.