Acclimation time is a compromise between two different concepts: on the one hand, you want to go as slowly as possible, so the transition from one set of water chemistries to another is gradual. On the other hand, the water quality in the bag probably isn't very good and if your octopus shipped from somewhere overnight it is likely that there's ammonia and CO2 present, so the sooner he's out of there the better. In the end, I usually take 45 min or an hour for something that's been shipped, and a bit longer for something that I picked up local.
Try to keep the room lights low during acclimation. I suggest being present for the whole process (My wife acclimated Mr. Octopus because I was at work: he would start climbing out of the bag whenever she left the room!). This is a very stressful time in the animal's life, so don't bother it. When you let it loose in the tank, let it go at its own pace and leave the tank lights off for the day.
Dan