I've hit 24 hours twice on accident shortly after coming home with a newborn child, and was seeing pulsating walls and other weird visual stuff and generally feeling pretty shitty. The first time, it took me until much later to realize I had been awake so long. So definitely some YMMV.
“Fireflies” in my vision and then peripheral movement are the two effects that hit me first, usually around 24 hours.
I will often hear what sounds like voices from another room. The first time this happened I freaked out a bit, searched the entire house, peeled outside through windows. When it was clear, I put my ear close to my radio thinking it was on at a really low volume. But of course no matter what I did the sound stayed the same. Then the irrational state of my mind lead me to believe the house was haunted.
Audible hallucinations and peripheral movement are exactly what happens to me - I rarely am awake for more than 20h, but it's been 18-22 hours every day for 6+ months, and eventually they just sort of end up there regularly.
