When you do something you usually only do alone, like sing while washing the dishes or making food for example, and someone was standing at a distance in the same room and you notice, so you ask them how long they've been there.
guy 1: *singing*
guy 2: hi
guy 1: how long have you been standing there?
What you say when someone tells you something he thinks is hilarious but really isn't. You say this to avoid hurting your friend's feelings in breaking it to him that his joke sucks.
Dudeman 1: "Oh man, I was playing horseshoes in Red Dead Redemption and I got a really close throw and John was all like, 'Did you see that!' It was so funnydude!"