What is your longest time spent on solving bug? And what was that?

Actually series of bugs 😬

I had this guy in my class who was in a team with me and two others after a day of coding he had a huge pile of trash and it was faster to recode from scratch than to fix the mess he had made

There were plenty...

Recently, I and another developer spent a couple of days trying to understand on why the Oracle won't create an ext table and throw an ORA.

Finally, we've discovered that the Oracle newer version has some bugs with dba directories.

Years ago, I spent a week solving a nasty multithreading bug in some C++ code. Debug build just ran fine, whatever I threw at it. Needless to say, only the Release build crashed randomly. But now, I no longer could debug it 😩
That took a long time of careful reasoning about the code to fix it.

Couple weeks maybe even a month for me, cant exactly say what it was but it was a bug that displayed no reference errors and the api said nothing about what was going wrong, found it in the end though, slept so soundly that night.