Not the longest but the most memory of.........
In the early to mid 1990's I played a local Very Good Kansas City player, this is just as I was getting better at nine ball. We played for over 12 hours races to 5 for 50 a set.......
In the first 3 hours I was stuck easily over 400 dollars and I was getting the 6 and the last 3 I think

Believe it or not, I could run out at that time, I was a lower B player but the guy I played never missed a ball and if he did he hooked me.
After several hours of getting tortured though, I knew I would win because of my youth and desire, I was betting on this older gentlemen (mid forties i think) to tire first..... he did, and I got in stroke finally after being able to hit one or two balls in a row
I ended up when all was said and done about 450 winner

Needless to say when I quit I went home and slept for 12 hours then drove back to Florida where I was from a preceded to beat every one that had been giving me weight previously.
The moral to my story is what I have tried to tell many of my present friends. You have to play some very long sessions (season yourself) I believe to get better and get very comfortable playing for cash...
After several hours I never even thought about the money at all, I was completely engrossed in the game and my level of believing and confidence soared from that point on...I had cleared a plateau in my game and moved up a few notches just in confidence alone, not counting all that I learned and the better I became from playing so much....
So play on, get in some long matches and put alot of pressure on yourself for a long period of time and win or lose you will notice a positive difference in your game in the long run mentally and physically......