Apparently a lot of people come to me for their physics experiment information, so I have an update. The pitchdrop experiment is listed as one of the longest running experiments ever. It surely must be one of the slowest.
Pitch, like glass, is a very very thick liquid that can be shattered and that most people think of as a solid. This experiment has been measuring its viscosity by timing the drops... they fall about once every 8 or 9 years. There have been 8 so far, most recently in November 2000, and no-one has ever seen one fall - that is, separate from the funnel. There's a webcam pointed at it, but it should be a while yet before you need to start monitoring it.