Been there done that. Collided with the Satellite in orbit around Sag A 
To get back to the topic:
Personally, 4.5 hours is the limit for me, flew from Stockholm to Belgrade in one go in a C208, now I’m doing shorter flights now and then and have reached Timbuktu in Mali. I didn’t spend those 4.5 hours looking through the window all the time, there was youtube, music and work too. And yes, i have more than one computer. No one is forcing me to sit and stare at a virtual instrument panel.
Doing a lap of Oz in the C152.
No AP there! Trimming is handy, but it starts at load management too. One beefy pilot, and no PAX, means a constant drift to one side. Almost finished too.
In a plane with no AP, there is always something to do. Even if it is looking at approach templates, or a quick calculation of sink rate required.
I really thought playing this sim for more then 4 hours is not only harmful to the player but the computer too. You are going to replace your computer very soon due the the graphic card over heat and damage the other hardwares. And it’s not worth it at all!
Best for practicing landing .
Not a big deal. I did the longest flight in the world in P3D. Do you really think I sat staring at my PC for 18 hours?