[neilk]

I personally find these examples underwhelming. Most of them are processes that require time, like the pitch drop experiment.

I suspect that the things in our lives that truly have value and take a long time aren’t easy to identify as projects. No one person starts it with a clear idea of where it will end. Investment in future capabilities. Knowledge gathering without clear application or business model. Strengthening institutions and traditions of human rights to ensure that no one group can arrest history.

[Timwi]

Depending on how you draw the line, it could be argued that science — the project of uncovering the workings of the universe — is the longest-running of all. Although the word “science” isn't that old and is generally associated with the Age of Enlightenment, the desire to understand the world goes as far back as humans can think.