Probably around 9-12 hours when I was doing a marathon of all the SNES, PS1, and PS2 Final Fantasy games roughly 17-18 years ago. Was still in my 20s. Would come home, get some exercise and dinner in, and then play from like 6PM until anywhere between 3AM and 6AM, get a little bit of sleep, and then go into work by 8AM and function fine. I could never do that anymore!