Longest computation that your calculator performed for you?

[eried]

For me it is 20 minutes

I was in the early physics courses in the university. The exam's problem was to determine the best angle to shoot something trying to hit a precise spot having some simple external conditions. As I usually did not went to any non-programming related classes I had no idea of how to solve this.

However I had my excellent, paint-faded, keys-broken 49g+ with me, including the (optional) equation library to get the equations and then coded an ugly FOR loop to iterate in a range of angles I thought was optimal. Looking back it was probably the worse thing I could do, but I continued with the other problems and after a long while I had the answer... too bad that I had to explain to the teacher why my answer was like 40.022 instead of the expected 40 and I did not had the normal explanation either. But I got half a point for the approach