For me, it was literally 3 days.
It began on September 11, 2001....we all know what happened on that day.
My girlfriend was working part-time as a security officer/receptionist about 5 blocks away from there in one of the Salomon/Smith Barney (now CitiGroup) buildings. Now, being a security officer, one cannot leave the job site at the end of the tour unless properly relieved by another guard....but when the Towers were attacked, no one was allowed into that area, hence, none of the relieving guards were able to get to her job site, and she ended up being stuck there in Manhattan for three days.
I, however, didn't know this at the time, and she couldn't call me because phone service in her area was knocked out. So I ended up staying awake for 3 days sick with worry, as I didn't know what had happened to her.
I tell ya, staying awake for that long is NOT fun, especially if you're worried about something. You're exhausted and your thoughts lack cohesion at times. I wouldn't wish it on anyone.