Well, in High School, my first girlfriend found out when my birthday was. I told her I didn't like celebrating it, and she kept pressing that she was going to surprise me then. I told her again that I didn't want her to and she persisted. In a wild attempt to gain ground, I sent her a text saying "What if I tickle you until you promise you won't?" And I got a text back that said "Bring it on. 🙂" The next day, I picked her up so we could hang out, and as I opened my passenger side door I said "Remember that text I sent last night?" And then proceeded to tickle her. I'm not going to say it lasted for hours, but it was a rather extended period of time taking into consideration this wasn't planned. If I had to venture a guess I'd say more than ten minutes, less than fifteen. The odd thing was she never once said stop. There were a couple breathless "Okokokokok"s in there, but nothing telling me to quit. After a while I started feeling bad, so I asked her if she gave up... And she still said no. I have no clue, to this day why I stopped.
But hey, I got to tickle one of the cheerleaders of my school for a while, so there's something.