Never - the last time was a few years ago. I stayed up all night a few times when I was taking classes, and a few times in undergrad before exams. I need my sleep and no one's going to interfere with that.
I've never consciously pulled an all-nighter or anything like that.
I've only slept for 2-3 hours for reasons like weather (too hot/cold), having a stuffy nose, or stress - but I'm always in bed in those cases. Aside from things like that, as well as emergencies, I don't see why I would ever choose to skip sleep.
No more than a few times a month and I do it for no particular reason. If it's past 12 PM and I'm still awake, then I stay up because I don't want to wake up in the middle of the night. I usually wake up in the late afternoon and then eat dinner afterwards. I don't even eat breakfast and lunch because of my messed up sleeping pattern lol.
It depends. Sometimes quite often, sometimes not. I often end up doing it when I screw up my schedule so that I can get it back on track quickly by just staying up until around the time I want to go to bed the next day.