My longest session has been 7-8 hours, but thats only because I am forced to stop for whatever reason. If I have a good idea that takes longer to forge I'd got at least 12 if I could. This creation is what I thrive on, there is no time limit!
Think the longest I went for was from about 8am til around 7pm or so. Not much, but its the longest Ive gone for. Usual sessions now last for around 3-4 hours before I get bored and play some MM or watch a DVD. The only time I really spend lots of time Forging is when I first start Forging an idea and want to get as much done as possible before that gem leaves my mind. After the initial layout is done Ill usually turn it off and come back to it another day.