Around 14 hours. I was working on a very difficult (at the time, this was my intro level course) c++ project my professor assigned. No one could finish it despite the many hours we put in.

What was it?

@Orni remaking an old text based adventure game. It would be pretty easy now, but the catch was that it was our first project using classes AND overloading operators AND writing makefiles to compile multiple files. We had 5 days to do it and we were still learning how to overload operators in class.
Honestly I'm kind of ashamed of not finishing it because I had it done a week later but would not get credit for a resubmission so late.