In my time on this board, the longest time I’ve seen for one question that does have a definate answer is the current May 33rd question that has been bouncing around here for about 2 months.
The origin of the 
logo was a long process for Cecil to work out, so that’s a good candidate also. What questions did either Cecil or the board get the answer to, but only after a really long time?
Here’s one that I read Cecil’s column, each week, hoping he’ll answer.
If anyone bothers to really answer this (below), then it will be the longest on this board by time it’s answered:
On what date did the world end?
Not really a trick question; I do mean the world where we leave.
'Course, if the world ends tomorrow, then I’m wrong – this won’t be such a long awaited answer after all!
In one of the books (damned if I remember which, they’re downstairs and I’m not) Cecil mentions that it took several years to get the Dope on a question re: mysterious “half-rings” that telephones would emit - turns out the phone company was doing line testing and some phones were more sensitive than others.
–tygre