About 3 hours at a time, sometimes more than once a day, when I was in my 20s. One really funny memory: when I had bought my first VCR and had some hot adult videos, I was into a marathon jack session when some woman called trying to sell me on investment services. At that age I hardly had any monetary assets, not even a car, etc. so I was wondering why she kept talking to me for such a long time after I told her how little I was "worth." But she kept asking questions, almost struck me as more flirting than trying to sell her services.
I was continuing to masturbate and had the tape running the whole time with all the "aaah's" and "fuck me" and other audio up full volume, but didn't realize until many years later when I heard my wife pick up the phone in another part of our house and with the tv in the background just how LOUD and CLEAR a tv can sound in the background when you're on the phone! I had assumed she couldn't hear any of it. I wonder if she could even hear the squishy sounds of my lubed hand on my cock. I crack up thinking about that conversation, and what that woman must have been thinking.