I stayed awake for two weeks once. After the first 8ish days you start hearing and seeing odd things. Really helps with music production. Ventured into soundscapes I'll never come back to.
I've got no clue how people stay awake for more than 48h; just last week I fell asleep in front of the computer after being awake for 24h. For me, it requires mental gymnastics to be able to go beyond that.