The longest session I had was 15 hours and it was exhausting.
It happened because I entered in two satellites after playing tournaments for 5 hours already, but busted in most of them quickly and did not want to one-table. Eventually, I won one of the satellites, and had to jump in another tournament. Ironically, I had a deep run there, hence the length of the session.
Now, I never register to sats or tournaments after a certain point, because playing when you are tired is not fun, and actually quite minus EV