One of the photographers that I'm intrigued by is Michael Wesely ( https://en.wikipedia.org/wiki/Michael_Wesely ) who takes ultra long exposures (a two week long exposure is "short" - his flower photographs are in that range). The museum of modern art was about two years. http://www.amusingplanet.com/2010/08/unusually-long-exposure...
You can see the tracks of the days and the cloud cover in those photographs. Note that the multiple year ones have multiple days going over a single track.
Aside from the extreme reciprocity failure in the film, its "only" about three stops from a one day exposure to a one week exposure... and another two stops to a month, and then another three and a half stops to a year.